Addtional words in incantations for Harry Potter?

How come in Harry Potter, do they use addtional words in the incantations for spells? For example, in the third book Hermione says the incantation Bombarda when recuing Sirius from his cell, and in the fifth one Dolores Umbridge says Bombarda Maxima when blow up the wall/door to the Room of Requirement. Also in the third one, at the beginning Harry, when under the sheets in his rooms, says Lumos Maxima instead of just Lumos like usual?