In any job, especially retail, youll always get a mixture of people - some who dont get paid alot and dont really give a damn, and some who have passion and knowledge for product.
As was mentioned before, Animates staff are trained in all areas - fish, dogs, cats, birds and small animals - of course its extremely rare for someone to have owned everything and hence have that much more experience over some training. Youll get someone serving you in the fish room, and you may think they are 'useless' but when it comes to dogs for example, you might find them really impressive.
If you know more about fish than the person serving you, thats nothing to be annoyed about really, just means you should be proud of the knowledge you possess