The most likely explanation for this is that the student was (a) not paying close attention to how much time s/he had actually spent on the test, and (b) was using the "back" button after previewing.
Ah ok thanks. So I'm not crazy. I think all my exams are on one web page, but, isn't it possible that an exam may span multiple pages? Isn't it inevitable that students will sometimes preview their answers and then want to make changes before submitting?
I presume your response will be that Webwork provides links to make the
back button superfluous? Even if that were so, students will instinctively
use the back button since that is what they are used to right? Isn't it an uphill battle to expect them to alter their web habits for one web site?
It is possible that an exam may span multiple pages, but I think that the expected workflow will be the same for multi-page and regular exams. My expectation for student work on a problem in WeBWorK, or on an exam, is that they will work on it, preview, continue work, preview again, and when they have an answer that they want to submit, will submit it (or, for a multi-page exam, advance to the next page). In particular, because the student's work is all provided on the preview page there is no need to return to the previous page using the back button.
The question of whether students will automatically use the back button is another matter, and I don't know if that is instinctive for them or not. In general use of the back button in WeBWorK (or any other application that is dynamically generating pages from server data) is strongly discouraged, because it returns to a page that reports what may be an out-of-date system state. My general guess would be that some students expect to use the back button and many others do not.
Does that make sense?