Hello niteshtripathi!
First, take a look at this picture:
View attachment 140095
From this picture, you can see that the 'fast' for FF and FS corners are not at the same coordinate.
So you can not expect that FF and FS for the n-type lead to the same results.
This is because the FS is a mixed mode corner so it does not take the full variations of the fast-fast or slow-slow process corners.
I hope my explanation was enough for you, but if not, you can check the model guide of the technology you are using for further information.
Plus, you can take a look at
https://edadocs.software.keysight.com/display/iccap2012/Introduction+to+Corner+Modeling
Best regards.