I think your store may have misidentified him: look at this site and scroll down (or ctrl+F) to "Heart Tails." Notice the long, almost sailfin-like dorsal yours has? Very similar to the double-tail in the upper picture, as well as a similar coloration. But his tail more closely resembles the heart-tail in the lower picture.
You might have gotten a bargain. Either way, he's very pretty. :grinyes:
very nice betta!!...i have a blue/read veil tail(long and droopy) there were no other variations at my fish store but it still a very fine looking fish...hes loving his new home and keeps making bubble nest...so cute