When I open the creative inventory, opening the 'Decoration Blocks' or the 'Redstone' tab, will cause all the blocks (including the ones in the category you're looking at) to have their light come from the oppisite direction in their icon. Normally the darkest side of a block in the inventory display would be on the right, but then it's on the left.

UPDATE: I found out why it happens in these two tabs. When any of the three types of chest is displayed (as item in your inventory) on the screen, the lighting changes to the oppisite side. I presume this has to do with the different hitbox a chest has, but I'm not sure.