The difference in lot size here is because NSE in March 2021 had announced that the lot size for all monthly Nifty Futures and Options contracts will be revised from 75 to 50 from July expiry onwards.
While the lot size for weekly expiry contracts will be revised from 75 to 50 starting August 2021 weekly expiry. Hence, there is a difference in lot size of July Nifty Futures and weekly expiry Options. If you check lot size for the August weekly expiry, you will see the lot size is 50 
You can refer to this post which explains all the changes in detail.