19 is the "hard" ceiling of the test, there is no way to score higher than that. At a certain age your child might need 23 items correct to score 19, they get 19 whether their raw score was 23 or 37, but the kid who got 37 right obviously actually does know more than the one who got 24 right. Extended norms help to allow for those kids who may actually have scored "more" than 19. The tricky part is that sometimes 18, and very occasionally even 17 is a "soft" ceiling - the child ran out of test before the stopped being able to answer questions even though they did not get a 19. Maybe the made some simple mistakes early on, maybe there was a time penalty. Maybe they got a few 1/2 if the test is one that allowed two points for some questions. It's a strange system how it's scored.
My DD did the WPPSI (the version for younger children) and had more than one 17 that was actually a ceiling score, one of them it simply wasn't possible to score higher than 17 at her age even though she completed the whole subtest perfectly with no errors and no more questions to do, one of them she had some early wrong answers but was going strong towards the end... That sort of thing.
