A little Googling has turned up that an ounce is actually a measure of more than just those two, but those are the most common.
Seriously though, wtf?
I’m looking at a recipe and it calls for “a 14oz can of sweetened condensed milk”. I’m thinking to myself - that’s one big-ass can! Milk being a liquid, it should traditionally be measured by volume. That’s a LOT of condensed milk. I look at the cans of condensed milk I have and they say 300ml. They’re also way smaller than 14fl oz. Great. So I start to think about all the cans of condensed milk I’ve seen and they’re pretty much all the same size (not counting the industrial sized ones). But you can’t convert mass to volume directly - not with a substance like condensed milk, anyway. If it was water, sure - that’s easy. Even regular milk in sufficiently small quantities is close enough. But who knows how much 1ml of condensed milk weighs? I set my scale to Imperial, weigh the can and it works out to 1lb, 1/4oz. How many ounces is that? I have no fucking clue. I use a real measuring system - you know, the one the rest of the world uses. Switch it to grams and it’s 464g. Ok. My trusty online conversion calculator tells me that that equals 16.36oz. 1oz is equivalent to ~28.4g, so subtracting the weight of the can that puts me in the right ballpark. It looks like a “standard” 300ml can in Canada is equivalent to a 14oz (by weight) can in the US.
But seriously - why in the hell would the same word be used for completely different units of measure - and especially ones that aren’t approximately the same?
