Hi @Samiksha_47544883 -
Thanks for the question. I’ve written about something similar here when I describe the differences in IV across various brokers and websites.
The Theta difference between Upstox and Sensibull is likely due to the assumptions in the Black-Scholes model that generates the option Greeks. This is exacerbated by it being an expiration day.
Here is the modeled theta for the Nifty assuming a strike of 22500, underlying price of 22500, IV of 12%, and with various time until expiry. As you can see, theta has exponentially larger impact near expiry.
We are using discrete, intraday time intervals to calculate our Greeks including IV. If Sensibull or anyone else is using something basic like “1” (or a larger fraction like 0.5) for one day until expiry, you will get a different theta. The orange bubbles in the chart above show intraday values of theta. You can see that small differences in time on expiry can have a substantial impact on theta.