Difference in margin requirements for equivalent positions

Hi, I think the following two sets of positions are equivalent.
(a) Sell 2 lots of Nifty FUT, and sell 4 lots of 20800 PE
(b) Sell 2 lots of 20800 CE and sell 2 lots of 20800 PE

But it seems that the margin requirements for (a) is much higher than that of (b). Does anyone have a good explanation as to why?