:?? It sounds to me that maybe you were 1/4 of a round off? It can happen to lose track sometimes. Does your decreases follow the sides of your foot? If not, it means your stitches were probably not arranged the way you thought they would be. Otherwise it could simply be due to the way the stitches were arranged just before you started doing the kitchener stitch.
It shouldn't be too hard to undo the seam and go back to where things start to go wrong. You should be able to try on the sock at this point, and this would help you to identify your stitches better. Make sure the decreases follow the sides of the foot, and that when you're done decreasing both needles should follow your toes horizontally.
You're almost there, keep at it!