I'm sorry. Just because a registering body says there is a difference, doesn't mean there is one. Isn't this the same body that only recently accepted double dilutes but was fine with single dilutes?
Chestnut and Sorrel are the SAME colour genetically. Both would test as ee.
The AQHA will register them as different colours though, sorrel and chestnut. If it's the red-orangey shade, they get called sorrel. If they're technically liver chestnuts or just more brown, they're chestnuts.