That is different, though I would not have seen it unless i was looking for it. Not difficult to see how the store got them mixed. The dwarf lettuce actually appears much closer to Frogbit (Limnobium sp.) than it does to Pistia stratiotes.
Now I wonder if anyone knows the scientific name? There doesn't appear to be one.
The true water lettuce is Pistia stratiotes
, and this is the only species in the genus. It was described and named by Linne in 1753. Linne is the more common botanical form of Carl Linnaeus, the Swedish biologist who first classified all then-known species of plant and animal and in doing so invented the scientific nomenclature system we still use today to classify all species of life on earth.
Kasselmann says Engler described 4 varieties but these have no taxonomic value; she has no reference to the dwarf form. The Plant List, a listing of all known botanical species and "the" authority on the subject lists 26 synonym names but none are distinct species.
There are photos of this plant on some of my plant forums but they simply call it dwarf lettuce or dwarf water lettuce. It would be interesting to know what it is, and how it came to be.