Hi ... As far as I know, this has something to do with STT tax which is usually charged higher for all ITM on the day of expiry. Therefore, it is must to exit the ITM options before they expire (before 3:30 PM on expiry day). Also, because of the fact that traders are pretty much aware of this hefty STT on expiry days, hence it affects the total number of buyers and sellers in those particular ITM contracts and therefore we get to see changes in intrinsic values. A person once told me this thing long ago, I am sharing with you whatever I remember. Thanks.
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