Jamieson, Fausset and Brown on Exodus 12:30

30. there was not a house where there was not one dead--Perhaps this statement is not to be taken absolutely. The Scriptures frequently use the words "all," "none," in a comparative sense--and so in this case. There would be many a house in which there would be no child, and many in which the first-born might be already dead. What is to be understood is, that almost every house in Egypt had a death in it.