The reg auction had plecos and cats too. Vendor table selection depended on who registered first and what they had on hand. Durkins do lots of plecos and cats so that's why their table had a lot. Others sold other items.
Auction selection in general varies a lot depending on what people bring. Sometimes old world cichlids, sometimes plecos, and sometimes new worlds are the big thing.
Oh, and no dry goods at the MCA auction (at least during the last year I've been aware of the group)...with the exception of the vendor tables.