The background contains vertical bars 2 pixels high. The edge of the icon should therefore be positioned so that only 1 pixel of the vertical bars is showing rather than having both pixels showing. Also the left and right edges should have the whitespace between the vertical bars showing rather than the vertical bars directly adjacent to the left and right edges.

Also, I'm only seeing the inconsistency in the concave parts of the icons. The outer border is fully consistent with the design guidelines.