Never really bought many slabs off there, but one of the first dealers I bought from stated that CGC’s grading is subjective and to be aware of the possibility of confusions like this occurring, especially when distance buying with limited visual information.
There’s a basic level of consensus about presentation and permissible defects, but it isn’t cut razor-sharp on either side of the transactional argument.
If you can’t handle this level of confusion with the facilities available on a certain platform, then looking for an alternative stratagem elsewhere is simple common sense.
It’s a learning experience for the buyer, in this case.