>that person who doesn't know the difference between WOTC's d20 System and TSR's D&D system
>that person who insists that AD&D, BECMI, and OD&D are d20 System games because they use twenty-sided dice
>some people think they're all part of THE d20 System
Do they? IME they just don't know that there is such a thing as "THE d20 System" and think it just means d20-based games.
Can't really blame them either; we have left the "d20 System for everything!" age, so now d20 is mostly just D&D.
True, everyone strived to move away from D&D in order to differentiate themselves, and the easiest and most superficial change possible was a change in the type of die representative of the system.
>everyone strived to move away from D&D in order to differentiate themselves
I don't think we're talking about the same thing.
There was a time, after the release of 3e and the Open Gaming License, where everyone and their mom made d20 System games because they're easy to write (badly) and sold well, and there was a big chunk of gamers who wouldn't try anything save d20 System games. (I don't know which is the chicken and which is the egg; probably just a reinforcement spiral.) This is no longer true.
It's not that people move away from d20 to differentiate themselves; there are very good reasons to move away from d20. (tldr: high variance and flat curve, not the ideal foundation to build a system around.) They just had an incentive to go d20 anyway, for a while.
To add to Anon's point, there d20 adaptations of World of Darkness, Call of Cthulhu, and Big Eyes Small Mouth, despite all three of those having almost decade-old original rulesets.
The 3.x era was a strange time.
>Someone, somewhere, used a term outside of the highly specific context OP cares about