With genetics you usually get some babies that resemble the mother, some that resemble the father and some that fall in between. With fish you usually get a few oddballs too that don't resemble either parents or anything in between.
If both parents have long fins then most of the young should have long fins. With Bettas some will probably have short fins and this is from their original wild ancestry, (wild Bettas have short fins).
It's like breeding goldfish. If you cross an orange fantail with a white fantail, you get some babies that are orange, some are white, some will be orange and white, and some will be bronze. There might even be some black ones, depending on the fish's grandparents. The bronze ones are the original coloured goldfish. You will also get mostly fantails but some will have single tails, again due to original wild goldfish having a single tail.