Just a quick tangent, but I think this was the system used for the Cavalier class back in AD&D (but might have been AD&D 2nd ed).
Back then, an 18 Strength had percentile an extra bonuses on a table already. The Cavalier class added rolling percentile for certain ability scores, regardless if they were 18. And every level they would go up by like 2d10 (or maybe it was 3d8) percent. So if you had a 15/73 (how it was written because that's how Strength was written) and you leveled up (just one class, so no question leveled up in what), you would roll those dice and maybe have a 15/91. If it went to 16/xx, congratulations, you now had a 16 in that ability score. Back there there was no ability advancement from level, just from magic items, so this was a huge deal.
This was back when everything was it's own custom designed subsystem. It was quite the mess. It wasn't even until 3ed that the trumpeted having a unified mechanic of d20+mods >= DC.