I try very hard to not mislabel my plants, and I sure hope my sources (and yours) try as hard or harder than I do. That said, there is the fact that all the plants we purchase are labelled by a human, and therefore subject to mistakes.
I still think the plant I've been nurturing for a couple years as Double Mahoi (Double is s'posed to get 7'), is in fact a Super Dwarf Cavendish or other 18" tall cultivar and was mislabelled. Fortunately, I have not sold any pups of this plant, so the mistake will stop with me if it is in fact a SDC. I won't know until it fruits the second time (prolly another 2-3 years down the road from now) if it is a Double, but if it fruits at only 18" tall, then I will suspect SDC even more.
I am growing it in the same bed, and with the same care, and was planted the same time as my other plants that are getting over 5' now. The supposed Double is still only 18" tall with 3 pups...little tiny pups...
On the other hand, maybe I have a sport mutation of Double that will only get 18" tall. If so, then I'll have to come up with a catchy name for it. Maybe Super Dwarf Double, or Truly Tiny Mahoi, or Half-A-Double. Twice the bunches and 1/2 the size...whattaya think?