It's all down to the e... (Score 1)

Simple English language tutorial here:

When a letter is followed by an 'e', then you pronounce it as it you were saying the letter alone.

Example:

Route. Followed by a 'e', so the 'u' gets pronounced as U ("you"); clearly "root". Same with router.
Sour. No 'e', so it's "our" prefixed with "s".
Out. "Owt".

Since the Americans actually came from Britain in the first place, all these differences are clearly their own doing - I don't mean to flame, that's just how history has it.