Actually, if travelling on an "official passport", a visa might be required. All those who think a diplomatic passport is a good thing - it really isn't - other than the immigration lanes in JFK and IAD - they are pretty useless. You need visas for places you wouldn't with a personal passport. Not sure about UK -> Canada though.
Sometimes you just get stopped and selected and usually it's pretty quickly sorted out. I saw Al-Gore once get pulled out for manual search for a commercial flight while his two Secret Service agents were watching (maybe a year or two after 9/11)
I also saw an armed pilot once getting his gun back but his toothpaste wasn't allowed through...
It doesn't have to make sense all the time