They are imperial units. The trouble is that US Customary and imperial units are often very similar but rarely the same. The US pint (and gallon) are different to the imperial one; the US pound mass (lb) and UK ones became the same after the 1950s, but the "hundredweight" or cwt is different between the two – the US 100 weight is 100 lb; and in the UK it is naturally...112 lb. Of course, both systems are in rare usage in both countries (in the US, I understand in some industrial contexts; in the UK on some road signs for weak bridges). It is for this reason that "ton" is different, because, naturally, in both systems there are 20 hundredweight in a ton.

The whole thing is absolutely mad and most efforts to standardise the systems historically lead to the adoption of the SI unit system for "important" things, as well as a few good intentioned bits of legislation, like the International Yard and Pound agreement [1] which standardised those measures between the US and other former imperial users. Don't get me started on the survey foot, or the hell that arose when the units of classical mechanics, electromagnetism or similar scientific systems had to meet imperial or US customary units...

[1] https://en.wikipedia.org/wiki/International_yard_and_pound

And then there's the mile. Originally comes from the Latin for a thousand (mille) paces, and one pace being 5 foot, so a mile used to be 5000 feet. But now it's a beautifully round 5280 feet. How come?

Well, in the middle ages, the furlong was important, because it was the standard size for farmers' fields (based on how long a furrow an ox could plow without resting). But 8 furlongs didn't fit neatly into a mile, so to make things easier for everybody, the mile got expanded to a nice round 5280 feet, or exactly 8 furlongs.

I always wondered how they got their oxes to need resting at exactly the same number of feet ;)

Maybe they were better at standardizing their draft animals than their distance measurements.