Different color/texture on a never used slot and a slot that had skill removed.

When I remove a skill from action bar the color/texture of that slot is different from the color/texture of a slot that has never been used. On below screenshots Q and E had a skill assigned and removed, R and T have never been used (in and out of town).

http://img577.imageshack.us/img577/5187/screenshot0003cut.png
http://img818.imageshack.us/img818/1263/screenshot0004cut.png