Among the reasons why it appears higher than forecasts is the unavailability of high-frequency indicators for services growth and private consumption, sectors that have exhibited strong growth in the July-September quarter, they said.

However, they believe it needs to be seen whether private consumption, which improved this time around, can retain its momentum since the GST cuts came into effect at the end of the September quarter.

The Indian economy reported a growth of 8.2% in the three months through September, showed government data released on Friday. Not only did it beat estimates by economists, but it was also significantly higher than the 7% forecast of the Reserve Bank of India (RBI).

Given that the surprise growth numbers came in two days after the International Monetary Fund (IMF) graded India's national accounts data a C-grade on an A to D scale, some have questioned the ability of the data to paint a true picture of the economy.

"I would not go as far as to question the data," said Gaura Senguta, chief economist, IDFC First Bank.

According to Sengupta, there are not enough high-frequency indicators available for people to track and forecast consumption and services growth accurately.

Manufacturing growth, she said, did pick up and while services grew faster than estimated, overall growth was more broad-based.

Sengupta said that GDP is calculated in two forms. First is the gross value added or GVA route where indirect taxes are added to GVA and the subsidies are subtracted to arrive at the headline GDP number. This is the 8.2% real GDP growth number this time.

Second is the expenditure route where consumption, net exports and investments are added to arrive at a number. The difference between the two methods is accounted for as discrepancies.

"We have better data for GVA than we do for the expenditure side," said Sengupta.

Interestingly, the domestic currency hit a fresh low on Monday despite strong growth numbers. Reuters reported that the Indian rupee slid to a record low on Monday, hitting 89.7575 during the session before closing at 89.5475, down 0.1% on the day.

Some analysts said they did not expect these GDP growth numbers.

"These days, GDP data is accompanied by a surprising 'What, how!' instead of 'Ah, okay, we saw it coming'. Let's be honest, we didn't see this 8.2% growth coming, even in the wildest dreams," analysts at Bernstein said in a note to clients on Monday.

"The first and foremost question that might spring to the minds of most of you is: Is it true? How would you explain the IIP growing at an average 4.1% during the quarter, the 8 core industries at 4.5% with coal, crude oil and gas showing a contraction while electricity generation showing a very slow 3.7% growth?"

Bernstein analysts said the problem with mapping these numbers with GDP (or GVA) and comparing has a basic flaw: inability to track services sector in high-frequency parameters, which is the lion's share of GDP and grew 9.2% in Q2.

What's worrying?

According to experts, the slowdown in nominal GDP growth —GDP at current prices—is worrisome. The difference between nominal and real GDP growth has also shrunk in the September quarter, to 0.5% from 1% in the previous three months. The nominal GDP growth of 8.7% was the slowest in 19 quarters.

Analysts said since calculations such as long-term growth targets and fiscal deficit numbers are based on nominal GDP, a slowdown will be detrimental for the economy.

Mint reported on 28 November that nominal GDP growth has averaged 8.8% in the first half of the fiscal year and is running below the 10.1% growth projected in the Budget in February.

Economists pointed to the so-called deflator, a measure used to adjust for inflation, being low due to benign inflation.

"When we look at GDP forecasts, we look at current prices and since inflation—both CPI (consumer price index) and WPI (wholesale price index)—are low, the difference between real and nominal growth is thin," said Madan Sabnavis, chief economist, Bank of Baroda.

He said that in the same manner, the difference used to be high when inflation was much higher.

"One of the reasons why the estimates were varying is the absence of high-frequency services sector indicators. Since the services sector was one of the main contributors of growth this time round, the estimates were divergent," said Sabnavis.

Some economists pointed to the goods and service tax (GST) cuts in late September and the initial announcement in August as reasons for the stronger-than-expected growth.

"The GDP momentum was clearly higher than our above-consensus forecast," said economists led by Pranjul Bhandari, chief India economist/strategist, Asean Economist, HSBC.

HSBC economists said that the GST rate cuts were implemented on 22 September, but the announcement was made on 15 August and production might have picked up in anticipation of a rise in consumer demand.

The other reason, they said, is that the lower-income states are starting to rise, even growing faster than the higher income states and this, too, could possibly explain the strength in India's growth momentum. "After all, national GDP is the sum of state gross state domestic products (GSDP)."

Not everyone is convinced, though.

"We are suspicious to what extent the GDP numbers reflect underlying economic strength," said Alexandra Hermann, head of Southeast Asia research at Oxford Economics.

In the previous quarter, an artificially low GDP deflator pushed up real GDP growth; this quarter, it seems to be the statistical discrepancies doing the job, Hermann said. Their calculations showed that those added around half to the headline growth number.