It looks like a problem with precision. The upper left is a region attachment? Is it positioned in the right place, so pixels map to integer world coordinates? When rendering are you mapping the skeleton's world coordinates 1:1 with screen pixels?
Note that a region attachment is positioned using its center. If your image has an odd dimension, the attachment needs to be positioned at an x.5
world position. Eg, if an image is 10x10 it could be positioned at 15,15
and the image pixels would match 1:1 with screen pixels. If your image is 11x10 you'd need to place it, for example, at 14.5x10
because the X dimension is an odd number.