The bishop on h8 must have originally been the pawn on c7. That pawn needs to have captured five White pieces to get there, all on black squares. There are a total of six White pieces which could have been captured on a black square - two knights, two pawns, a bishop and a queen. However the bishop which started on c1 never moved and must have been captured there, so that just leaves five White pieces which were were captured on black squares other than c1. Hence position 2 is illegal because in that one a White piece was captured on f6.