I'm performing a two-Sample Paired T-test VBM using a statistical analysis with randomise TFCE.

1) Results obtained by estimating  the 2 paired differences A-B and B-A showed to be different from each other (what was expected) but one was not the opposite of the other: why?

 2) More generally, how can one optimally choose the number of permutations in randomise TFCE?

Thanks in advance