Back wider than front?

I'm making a small (with added length). I discovered when I was measuring the (nearly) finished muslin that the back is wider than the front and the side seams are not at the side. Going back to measure the pattern, the back - from the fold to top of side seam - is 12-5/8 in, total 25-1/4 inches. The front - from the cut line to top of side seam - is 12-5/8, less 1-1/2 inches for a cut-on placket on each side, total 21-1/4 inches. Less another inch for overlapping the plackets for buttons, so the front is 21-1/4 inches wide. Could you tell me what I did wrong? Many thanks,