They're not actually 1/8th of an Inch either.

But it is in fact commonly called a 1/8" (headphone) jack.

As noted in my comment, never noticed 1/8" before (and am American, so no particular metric bias...). All e.g. MP3 players historically called them 3.5mm jacks, is what my brain is telling me.

Some googling:

3.5mm jack - 49million results

1/8" jack - 63 million results

3.5mm stereo jack - 19 million results

1/8 stereo jack 5.9 million

1/8" stereo jack - 5.47 million

Evidently, it's a common term! Guess once I decided what I was calling them, that's what I searched for, and what I skimmed technical details for.

Ammunition caliber is similarly interesting

Right .223 Remington has the same bullet diameter of .222 Remington; namely 0.224 inches.

Metric dimensions of rifle-cartridges, however, seem to reliably be the land-diameter: 5.56x45, 7.62x51, and 7.62x39 all have eponymous land-diameters.

And famously, a 2x4 piece of lumber is not actually 2" by 4" either.

Well, that depends. Prior to finish planing they are exactly 2" x 4", and that's where they get their name. You can buy these in bulk straight from the sawmill or lumber wholesalers. But most lumber is sold neatly planed and then the outside dimensions change.

TIL. I know that if you bought a 2x4 that was actually 2" x 4", it wouldn't fit into anything else that expects typical dimensions.