What browser are you using?
I am using Firefox.
To test whether I encountered what you experienced, I created a new list, added fourteen items to the list. I set the list to display ten items per page, then selected all the items on the page using the checkbox at the top of the list and hit the delete button. I was left with four items on the list, so it appears that only the first ten items were deleted.
To verify that the Watch List operated in the exact same way, I opened up each page of my Watch List in separate tabs so I could find links to the items in case they disappeared, then opened up the first page of the Watch List showing ten items per page, then selected all the items on the page using the checkbox at the top of the list, and hit the delete button. Only the ten items on that page were deleted. Then I added those items back to my Watch List.
