Chess puzzle / From Smullyan's Sherlock Holmes

Hello,

I think I have found some mistake in one of puzzles in the book (from subject) it is about "You really can't you know".

When we have conclusion that a3 made a move from a4, why couldn't pawn make a move from b4 by capturing something on a3 - taking under consideration that last move was white knight (for example from c2 into b4) ? - Then a3 pawn could previously capture black bishop on black squares and d2 could be captured by any black figure.  

P.s. (sorry if I did not paste the problem here - but I do not know if I can based on copyrights) 

Please advise what I am missing.

I found another! He said there was only 1 legal variation. I found 2 !!

It's fine to quote chess problems - there's no issue with copyright, especially if you mention the source as you have already done.

And I agree, that retro problem seems to be unsound for the reason you gave. It could be corrected by specifying that White made the last move with the bishop instead of the knight.

Or that the situation shown is before knight move, not after.