In my opinion it is probably something used as a cost-saving trick. Maybe some connectors/sockets/boards just happened to be cheaper that way at the moment. If the manufacturer do not care about being standard-compliant, you get something like that in the process. I mainly (only?) see such things in a more or less cheaply made “no-name” chinese-imported electronics.
Yeah, that is one explanation. But the magewell products are expensive and there have good linux support (only reason why I picked them), which is usually not found in cheap chinese products (even if I know magewell is chinese). I thought a bit more about it, and it seems they have have carefully selected an A type connector, in the documentation, they state we should not use an other cable. Maybe they have some electronic in their cable and used the A-A trick to prevent users from using another cable. I could take the cable to the electronic lab to have it tested, but sadly I don't have spare lying around and the devices are all in use.
Looking at the height of the chasis, fitting the B connector there might have been harder, and they likely didn't want to have a fixed cable.