Customers perceive that they're being avoided or ignored. When customers enter the store, if sales people don't actually see them walk through the door, they often assume these people were greeted by someone else and don't approach them. In many cases these customers were greeted by no one. Oddly enough this can happen just as easily on a slow day as on very busy days like weekends. In fact, customers are even more likely to be missed on slow days.
Customers seem to be sales person magnets.
This second situation is the exact opposite of the one where customers feel ignored. We've all seen customers wheeling around angrily after being greeted the fourth time, declaring that yes they have been greeted. The inability to know who's been greeted and who has not evokes negative feelings within customers who have not even begun the sales process. Both situations can be extremely irritating and potentially harmful. I was in a store last week where a minority customer had been not greeted for the longest time.