The job of any retailer is to sell product. That said they are telling you the smaller ones can be kept in a smaller tank. What they don't tell you is how fast they grow and how soon you would have to move them to a larger tank. The best advice if you are planning to keep a fish long term is to ensure your setup is large enough to accommodate it's adult size and it's environmental requirements. Doing so you and the fish will be better served.
Here is kind of a guideline on tangs and tank sizing. Rest assured it is the bare minimum requirement but nonetheless you get an idea from it.
# Acanthurus achilles, Achilles Surgeonfish, 100 gal (380l).
# Acanthurus coeruleus, Atlantic Blue Tang, 75 gal (285l).
# Acanthurus japonicus, Powder Brown Tang, 75 gal (285l).
# Acanthurus leucosternon, Powder Blue Tang, 100 gal (380l).
# Acanthurus lineatus, Lined or Clown Surgeonfish, 180 gal (680l).
# Acanthurus nigricans, Whitecheek Surgeonfish, 75 gal (285l).
# Acanthurus olivaceus, Orangeshoulder Surgeonfish, 135 gal (510l).
# Acanthurus pyroferus, Mimic Surgeonfish, 75 gal (285l).
# Acanthurus sohal, Sohal Surgeonfish, 180 gal (680l).
# Acanthurus tristegus, Convict Surgeonfish, 75 gal (285l).
# Acanthurus tristis, Indian Mimic Surgeonfish, 75 gal (285l).
# Ctenochaetus hawaiiensis, Chevron Tang, 100 gal (380l).
# Ctenochaetus strigosus, Kole's Tang, 75 gal (285l).
# Naso brevirostris, Spotted Unicornfish, 180 gal (685l).
# Naso lituratus, Orangespine Unicornfish or Naso Tang, 135 gal (510l).
# Naso unicornis, Bluespine Unicornfish, 200 gal (760l).
# Paracanthurus hepatus, Pacific Blue Tang, 100 gal (380l).
# Zebrasoma desjardinii, Indian Ocean or Red Sea Sailfin Tang, 135 gal (510l).
# Zebrasoma flavescens, Yellow Tang, 75 gal (285l).
# Zebrasoma scopas, Brown Tang, 75 gal (285l).
# Zebrasoma veliferum, Sailfin Tang, 135 gal (510l).
# Zebrasoma xanthurum, Purple Tang, 100 gal (380l).