I have the following book:  http://www.paperbackswap.com/book/details/9781582346106-The+Line+of+Beauty

The actual book has 438 pages, but this page says 400.  Other paperback ISBNs of the same book say 501 for a couple, and 300 for another one.  That's quite the difference.

Do publishers who submit this data usually get these numbers wrong? 

If the ISBN#'s don't match, then it's a different printing and therefore the page # might be different. Some books have really tiny type face and need less pages. Others have bigger type face. Some editions might have a lot of half filled or blank pages, or little pictures for each chapter. While others are just the text.  If the ISBN matches and the page number is different than it's probably a mistake.   But if it the ISBN doesn't match, it's most likely right.  So it just depends on the type face and how much extra stuff they add. 

ETA: some of these different versions might even have the same cover.

Sometimes publishers also publish multiple editions under the same ISBN, and this can cause a problem with the page count -- for example a trade paperback and a large print title, or an ARC (which aren't "legal to trade here") and the full-release edition, or a trade paperback and mass market paperback can be the same book, same publisher, same ISBN, different numbers of pages. 

In this case, however, I suspect it is an "Amazon issue".  Amazon, who is often "loosey goosey" with book data, lists the book at 400 pages, while WorldCat, Alibris, Biblio.com and you note that it is actually 438 pages.

If there's a significant difference, you can always hit the "Edit Book Data" link at the bottom of every page and update the information.

Cheers,

Catt