If Mary Poppins is really only the 38,085 words
this page says it is then 4ish hours doesn't seem off. Hard to go by page count, especially with a children's book, due to variations in font and margin sizes, illustrations, etc.
I've not questioned books too often, but do remember one book that was missing stuff. That one though was an actual error where a chapter cut off a few paragraphs early and was later fixed when reported. I've also run into at least one where the reading doesn't match the book text in spots. It turns out that the publisher sent an earlier version, before final edits were done, to the audiobook producer and that earlier version is what ended up in the audiobook.