Seedless grapes are produced commercially by two different methods; grafting and artificial hormone treatment.
Grafting enables the growwer to take advantage of a good rootstock (perhaps a variety that is resistant to fungal infections and soil pests, but produces poor grapes) and a good fruiting variety (that is perhaps juicy, sweet and seedless, but is prone to infections or attack at the roots).
However, many seedless grapes sold in the shops are just ordinary seeded varieties that have been sprayed with artificial hormones to provoke fruit formation even though pollination has not occurred. This method is not usually available to the individual gardener.
Anyway, one of several things may have happened in your case:
-Simple mislabelling at the supplier - happens all the time, especially if it’s a nursery to which the public have access, and they use identification tags that just push into the sopil in the pots, even if they are tags that tie onto the plant itself, mistakes in labelling happen; mistakes also happen in the supply of nursery growing stock, so they might have labelled to the best of their knowledge correctly, but still be wrong.
-The grafted scion may have died (or been inadvertently overpruned out of existence) and the rootstock variety may have taken over.
-They might be ripping you off, although this seems least likely; if they’re going to go to the trouble of growing something to sell, why bother to spend all that time and effort deliberately growing the wrong thing?
I agree with Mangetout’s possible explanations with one exception.
Most mail order nurseries are reputable. Unfortunately there are exceptions, including places known to have sold inferior varieties misrepresented/ mislabeled as better ones. I’ve had this experience on rare occasion.
Then I’d contact the company. Assuming you didn’t subject the vines to stressful treatment (resulting in dieback of the desirable graft resulting in the rootstock producing inferior fruit) you’re entitled to replacements or refunds.
In my experience, the mail order nurseries tend to rip you off more by supplying teeny tiny little plants (of the correct varieities), not the huge strapping specimens they show in the brochures, but clearly YMMV.