When you go to a big chain of pet stores like pet world or pet smart, you need to use your common sense when it comes to purchasing pets or supplies for your pets. The workers are not all experts!! A good portion of the workers that I have encountered are teenagers who are working part time for extra money. If you need more information, find a valid source or do your own research.
In response to a previous poster...if you bought a dwarf hamster and the salesman recommended too big of a cage, don't you think you should have asked about a smaller cage?