Here are two photos took with the same settings at the same time of day. Focused in the same spot. The first one is made with a Sigma Sens 17-55mm (at 17mm), the second is using Nikon's kit-lens 18-55m (at 18mm).
While the right side of the photo is comparably sharp with both lens on, the left side is significantly less sharp and looks out of focus in Sigma (while still sharp with the kit lens). Look at the tree and number 10 to compare.
Sigma lens full photo (17mm):

Kit lens full photo (18mm):

Sigma lens right-half:
Kit lens right-half:
Sigma lens left-half:
Kit lens left-half:
Files: