speaking of strange pronunciations. Why is it the British people don't pronounce the 'ary' in words here? Words like: Library becomes 'libree', Military becomes 'militree', Commentary becomes 'commentree'. Sometimes when I hear an announcer on TV say 'commentary' it sounds just like 'Coventry'.
That's a question with either no answer or lots, depending on your point of view.

The first answer is that we
do pronounce the 'ary' in words! Except, of course, for those of us who don't...
And as to why those that don't don't, well, there is no real reason other than that was how people pronounced it when they were learning to talk. And this is the only real answer to any pronunciation question.
In general I think British pronunciation is more apt to drop sounds than American and I think a lot of it is to do with the speed of talking. On the whole we talk faster which leads to these sort of contractions. You see similar artefacts in some East coast US accents where they is a tendency to be 'fast talkers'. In the Mid-west and American South people talk slower so are less likely to drop sounds. Conversely West Country accents over here are slower and tend to pronounce everything (although they have a whole other lot of oddities!)
As we've veered off into accents here's a 'fun' thing for the Americans to try out on your British half: Say "Adam" and "Atom" to your OH without using them in a sentence or giving any context and see if they can tell which is which.