Funny thing. In all my years of working with RS-232 (which was almost a full time thing between 15 and 20 years ago). I never noticed that.
Seriously, the fact that RS-232 is NEGATIVE LOGIC (-v for "1") is RS-232 101.
It is so basic, it is almost always mentioned within 15 minutes of the start of any lecture on RS-232.
To admit that you never noticed that is to admit that you didn't do your due dilligence. There could be
a thousand reasons why, but realistically, if you didn't have to troubleshoot it you probably didn't have
a reason to know because if it works, then what more do you need to know ?
Just sayiin', that is pretty basic.