Registries are notoriously behind science for colors. AFAIK
, the difference between the "bay" (A) and "brown" (At) agouti genes is a relatively recent discovery.
It also makes a difference if the horse has one or two copies of At, with the horse being darker with only one (At/a) vs. two (At/At). At is also recessive to A, so a horse that is (At/A) will be bay, not brown.