From time to time, we run a bot to convert American spellings to British in the main namespace. It's a pretty nifty bit of programming, but it's far from perfect. So you still need to try to use correct, British spellings in your editing.
Errors in the category tree structure may mean that the bot occasionally strays into namespaces where British spellings are not required. In general, the benefits of bot correction of tedious spelling differences are thought to outweigh its inability to stick strictly to the appropriate namespaces.
The details of what this bot is checking for can be seen at Tardis:Spelling policy/user-fixes.py.