"i have rarely seen this b4 .----- "
Off topic, but I doubt you've ever seen it before. That position is impossible since Black's only legal last move was Kb8-a8 and in that situation the pawn can't be at a7 (if it was, Black would have been in check from the pawn after the move before as well, which is illegal). It's not important, but I just found your choice of words coupled with the position somewhat ironic and thought others may find it funny as well.
but its whites move mate in one
It can't be white's move, because black was stalemated two moves before or it moved towards the center (also 2 moves before) - allowing pawn to promote (which nobody would do). No matter which combination of moves were played, this position is indeed impossible. You have to consider that black's king can't just wait on one square, it has to move at every move, since it's the only black's piece on the board.
Try completing a mate with 2 bishops vs. King before the 50 move rule. King and Rook vs. King is EASY in comparison.