Lets say Nifty is trading at 11800 with a bullish bias. You want to buy a call option. Lets say below is what the IVs look like.
Strike IV
-- ---
11700 13.0
11750 13.5
11800 14.2
11850 15.0
11900 14.8
11950 15.4
You can see that 11700 and 11750 are in the money so the IVs are lower as compared to IV of 11800 atm. ATM and above OTMs will have higher IVs.. Ideally you should buy ITMs. But if you are looking to buy OTMs then you have to hand pick a strike that has the least IVs. If you are lucky you can find a close by strike which is slightly mispriced.
In the above example you can see 11900 ce is at 14.8 which is less than 11850's IV of 15. So you are marginally better off there.
IV's are price derivatives. They are as dynamic as price. With every change in LTP of a option, there is a change in IV of that Option.
So just because on a snapshot of data, if a particular strike IV is less than other strike IV, It does not mean that the particular strike is cheaper.
Option chain Of NSE website will update once in 3 minutes. In this 3 minutes, if there are 1000 trades in 11900 CE strike, it means IV of that 11900 CE actually changes 1000 times in that 3 minutes. But option chain page will just give a IV that is captured for one particular trade at the end of that 3 minutes.
So it is completely wrong to track and rely on NSE Option chain IV data ( at such micro level). Just use the data broadly to get the picture of IV levels.
Secondly, When you are tracking the ascending or descending IV's in Nifty, Please do it on strikes with 100 intervals. Due to low volumes in 50 intervals, the buy-sell spread is these strikes is comparatively larger than 100 interval strikes and you will notice these IV differences also due to that.