Mumbai: Investors were surprised on Tuesday when the Nifty 50 and BSE Sensex moved differently. Around 9:42 am, Nifty fell 100 points to 24,460, while Sensex gained 166.15 points to 78,805.18.
By 2 pm, Sensex was down 400 points, or 0.5 percent, while Nifty had fallen 330 points, or 1.5 percent. The difference raised concerns about a possible technical or calculation error.
NSE Explains Difference
The National Stock Exchange (NSE) clarified that there was no market anomaly. It linked the unusual movement to the new Closing Auction Session (CAS), introduced on August 3, 2026.
Nifty and Sensex contain different companies and assign them different weights. Therefore, changes in share prices can affect the two indices differently. Separate order books for normal trading and CAS can make this gap more visible.
How CAS Works?
Earlier, a stock’s closing price was based on the volume-weighted average price during the final 30 minutes of trading. Under CAS, the closing price of eligible derivative stocks is decided through an auction.
Buy and sell orders are first collected. The system then identifies the price at which the largest number of trades can be completed. This becomes the equilibrium price and is used to determine the closing price.
Between 3:15 pm and 3:30 pm, orders are not matched continuously. Investors may place, change or cancel orders before the system calculates the equilibrium price. Therefore, the index displayed during this period may remain unchanged briefly and should not immediately be treated as final.
Options Traders Warned
CAS runs from 3:15 pm to 3:35 pm, while derivatives trading continues until 3:40 pm. This timing difference can create a temporary gap between spot and futures prices.
Such changes are important on expiry days. Even a two-point adjustment near an option’s strike price can sharply affect its premium. Traders caught on the wrong side may find it difficult to square off positions at the expected price.
NSE said the first CAS attracted 515 trading members and orders from 56,773 unique PANs. Retail investors need not panic over short-lived differences between Nifty and Sensex. However, options traders should remain careful during the closing minutes.
