I've received a reply, directly from Random House:
Thank you for contacting Random House Consumer Services. We appreciate your interest in our publications.
The same files are sent to all. The individual eBook retailers compress their files at different levels to make them compatible with the output capabilities of their various proprietary devices.
I've also received confirmation from Amazon that this is the case. It's their fault.
As a result, I'm seriously considering getting an iPad now. :(