The log gives you a timestamp when the page was downloaded. It knows nothing about when the person actually looks at it. I routinely have the browser running in the background bringing down pages while I work on other things. I may start a download of a page, move on to email and then return to the browser 15 minutes later. Event if its on the top of the screen, it still may not be looked at.
Someone can walk in an distract the user with other items.
That can't be seen from the server. He is doing an experiment on the effects of hypermedia and learning. Specifically he wants to measure the time in task .