Yep, though I wouldn't call it arbitrary. The trim-to length normally targets the middle value of the SAAMI drawing's length tolerance for a cartridge case with the idea that if you miss that value it will be by ±some amount so that targeting the middle of the range protects you from errors in both directions equally.
Most rifle cartridge cases and revolver cartridge cases and some pistol cases have a SAAMI length tolerance of minus twenty thousandths from the maximum value (the one on the drawing). This is why most are trimmed ten thousandths shorter than maximum, as that's the midpoint in the range. But some pistol cases can't be that loose because they headspace on the case mouth, so you find a number of them have a tolerance of minus ten thousandths, so the trim-to is half that, or minus five thousandths.
The .223 Rem is a special case. Its length was 1.760"-0.020" in the 1992 SAAMI standard, which put the trim-to at 1.750". But that was changed to a tolerance of -0.030" in the 2015 standard, so now the expected trim-to length is 1.745". Most load manual authors have ignored the change and have stayed with 1.750". It's just so easy to remember "one-and-three-quarter inches", so why would you change?