There are more than 30,000 species of fish. NO ONE is an expert on all of them.
You maybe a genius on cichlids, which would logically mean you can successfully cycle a tank and do water chemistry, but when someone has a question about breeding rainbows you suddenly look like a nimrod.
I have 25 years in this hoby, an MS in Marine Bio and a PhD in ichthyology and I learn a new tid bit or two every day. The sheer quantity of information required to correctly identify every fish that comes through a pet store would be pretty impressive to expect anyone to know. Now keep in mind that no one in that pet store is paid big bucks, do you really think there is much incentive to try and learn them all?
Within this hoby we all have a niche where we are really good with the information for that topic. What happens when you get asked a question outside of your niche? You suddenly look like a clueless idiot, right? Go ahead ask me a question about S.A. Cichlids...... I'll pick my nose and wonder what the heck you are yammering about. I know NOTHING about them. Does that mean I know nothing about fish?
Remember that as you talk to folks at pet stores. Some of them really are dumber than nails, but some of them simply don't share your passion for ________ (choose the group of fish to your liking.)