Hi Rebeck10…I’m assuming you have used a theoretical fair value estimate to arrive at this conclusion of Bank Nifty being mispriced…
Few things that I can think of…

Your interest rate estimate may be different

There could be effect of dividend which you may have missed

Assumption on time rule convention such as  actual/actual, actual/360, 30/360 etc could vary between the market and you
Now assuming you have factored in all these variables correctly, a 50 point under pricing on Bank Nifty works out to a third of a percent, which practically is negligible when you consider liquidity, transaction costs etc.