I just had my crown done for my molar and my dentist left a small gap on the back of the teeth. Is this normal?

My dentist said that she kept a little gap on the back of the crown so it will be easier to clean under the gum line. It’s not noticeable but I can feel the edge of the crown if I touch it by my tongue. Is this normal? She said she learned that in dental school. But I thought the crow is supposed to cover the whole teeth to protect it.