In the creative inventory, certain items are grouped by type - i.e. the various wool colors. Several items types, however, are not categorized properly but are rather each a category containing only themselves.
Applies to the following items:
All tools and armors
Arrows
Carpets
Glass blocks
Glass panes
Beds
Saplings
Leaves
Most food items, with the exclusion of apples
Silly question: have you been on a server/multiplayer world right before this?