Makes sense to me.
Original Aztecs is super old and super cheap, so people either bought it ten years ago or didn't pay much for it, so it's not _as_bad_ as if it'd been a more recent release. On Aztecs it can probably have more of the budget assigned to its clean up, Aztecs could do with a cleanup itself, and tenth planet needs its budget for episode four.
bbc doesn't make a great deal of money on these classic dvds so it has to plan them out this way. if they were a big money maker we'd have had an animated Power of the Daleks years ago