When planing flexible stock (like a 1x1) the face you're planing will be greatly affected by the face of the material that's on the bench. If the bottom face has a hollow in it, you're going to have a bulge in the side you're planing. After planing off the same bulge four times in a row I finally clued in. Either shim up the bottom so that you can't cause it to flex when you press on it, or, if it's short enough, pinch it in the vise so that it's supported all the way along.
A big board isn't flexible enough to have a problem with this, but it was certainly a problem for me this morning, and it had me baffled for a good half an hour.