There was a historic thing called the connector conspiracy. It described the tendency of manufacturer to use the weirdest connectors they could find, so that you would have to buy their equipment instead of their competitors, without any of that pesky competition on actual merit.
Some historic examples include Dec's SASI, which is almost buy not quite SCSI, DecNet, which isn't quite Ethernet, anything from Nintendo, and Apple's AAUI which was exactly like the IEEE standard AUI in function, only incompatible.
So, now we wander on to modern electronics.
Historicly, the base size unit for all things electronic was the tenth of an inch. Thats the pin spacing on DIPs, dating back to the 60's when we first were capable of building them that small.
Of course times have changed, and we have things like SOIC and TSOP, which are significantly smaller, the pin pitch is now 50 mils, where a mil is 1/1000 of an inch.
Times have changed further of course, and now we have LQFP, which has a pin pitch of 0.5 mm. Which is also OK, a perfectly good unit, for a new and separate standard.
So here's the rant: I went looking for 1/8 of an inch earphone plugs. They don't make them any more. They make 3.5mm plugs. And if you go looking for 3/32 of an inch earphone plugs, you need to actually look for 2.5mm. This is a Problem.
Lets convert 1/8 of an inch and 3.5mm into mils... thats 125 and 137 respectively, an error of nearly 10 percent.
We have been using 1/8 and 1/4 of an inch earphone plugs for decades, trivially since the 70s, on millions of radios, and earphones. Creating a new standard is perfectly fine, but Do not go redefining ones which are in use.
Or perhaps I am unaware of the real dimensions. In that case, label the miserable connectors correctly, they can't possibly be both 1/8 of an inch and 3.5 mm. Look at the absolute mess that bullet sizing has become: a caliber is a firmly defined unit, yet 40 caliber ammunition isn't that size...
I suppose it would be good to note that a 2x4 is actually 1 and 1/2 by 3 and 1/2, and all those 2 inch film cores are actually 50mm here too...
Someone is lieing, metric isn't the magic bullet, and all I wanted was a nice small commodity 3 pin connector for a serial port on my LED button... GNAR!