Hey donkey.

Don't want to be a smart arse but there are actually quite a few more DMA speeds through the history of ATA.

THree for Single-word DMA, three for multi-word DMA and seven for ultra DMA.

The speeds you mentioned were all from ATA 4 onwards (Ultra DMA) and were the top speeds for those particular standards (ATA 4,5,6 and 7). If you're interested theres something to be reading about when boredom grips you, otherwise disregard this post