I read this tweet last night
which obviously makes perfect sense, but then I thought of an example where things get very odd
“The trousers are blue”
“The pair of trousers is blue”
But the sentences both mean exactly the same thing, refer to exactly the same item.
How on earth does that work?!
ineke.co.uk 
I’ve been having similar issues. I’m sure we were never taught the rules, we just picked them up?