Imagine having a sorrel baby, that you registered as sorrel because she was light, but after their first year they got dark liver-ey chestnut in color and stayed that way. Now you've got a chestnut (which could go either way) registered as a sorrel.
My neighbors had a horse like this ^-^ he was a dark chestnut, regestered as a sorrel, and his baby picture on his papers were a light, what you would describe as sorrel color.
I have always preferred to use the term Chestnut, It's all the same, anyway. Sorrel/Chestnut confuses me, so I've always called chestnuts chestnuts.
It's kind of like how western riders/classes use lope and jog, but english tends to use canter and trot, which is the correct name, it's been around forever, and my preference, even though I ride western.