Queens do not lay perfectly. Sometimes they might lay more than one in a cell and workers might dispose of the surplus ... usually by eating it/them.
And I guess a queen might lay the odd egg that is not sticky enough and which does not adhere to the cell base.
Then there are the eggs laid by workers which also get disposed of.
And then there are eggs laid in queen cups that are disposed of if the colony doesn't want a new queen.
And then there are the eggs laid in drone cells which are disposed of if the workers don't want drones.