I recently had custom teeth whitening trays made by a dentist. They do not cover all of my teeth surface. Is this ok?

I recently had bleaching trays made by a dentist. They do not cover all of my teeth surface. The tray only comes up between 1/2 to 3/4 up on the center two top teeth. Bottoms do not cover all of my bottom teeth either. He said that entire tooth does not need to be covered because bleach soaks in tooth and spreads. He said the gel whitens entire tooth even though gel is not placed on entire tooth. Please help. Is this correct? Also seem to fit so snug and tight that I can barely take them out.