Behaviour packs are sometimes "incorrectly" spelt "Behavior Packs" (which would be US English) in the UK English language file.
The spelling mistake can be found in the following translation strings:
menu.behaviors
progressScreen.dialog.title.behaviorAndResourcePack
progressScreen.dialog.title.onlyBehavior
progressScreen.dialog.message.onlyBehavior
progressScreen.dialog.message.behaviorAndOptionalResourcePack
progressScreen.dialog.message.behaviorAndResourcePack
resourcePack.available.title.behaviorPacks
resourcePack.available.none.behavior
resourcePack.selected.title.behaviorPacks
resourcePack.error.ingame.behaviorPacks
resourcePack.description.bottom.behaviorPack
Source: the en_gb.lang file inside the template resource pack (which can also be found as an attachment)
P.S.: Technically it's not a spelling mistake, but US-American English (which seems a bit out of place since behaviour packs are spelt the British way in some translation strings)
Oh, I think it because of difference of American english and UK english.