Donna Duck

Have you ever noticed how you can pop into a store for a few minutes just to look and maybe get some ideas for an outfit, and because you are not buying anything that day, every two seconds, someone asks you if they can help you. BUT if I go to a store with every intention of buying something, and I need help, I can wander that store looking quite helpless and pitiful for quite some time, and no one will ask if I need help.
What is with that?