The Story of Jeremy and Jane

This story really did happen, but I’ve never been able to fully get my head around it.  Was I the ‘victim’ of a good natured student joke?  Because the obvious explanation is just too hard to believe. Perhaps you can help me decide.
Last Spring, I taught an introductory algebra class at a local institution.  It met twice a week from 5:30 – 6:45.
One of the students was a pleasant young man just out of high school – we’ll call him Jeremy – who was personable, caused no trouble, but never seemed too terribly interested in the class or the subject matter.  He had all the obvious symptoms of taking a class to satisfy a requirement and move on.
Jeremy always attended with his female friend – we’ll call her Jane – who exhibited similar symptoms, but not quite as pronounced.  They seemed to try to help each other stay motivated in a subject they’d seen some of before.  They were a team, for better or worse.
Jeremy and Jane almost always arrived with about 3 or 4 minutes to spare and were usually ready to go when class began.  They always tried to (appear to?) exhibit effort during ‘required’ self/group work periods, but the minute I allowed the class the freedom to leave if they had no more questions, they were usually gone.  (Others stayed, worked, and asked questions.)  They seemed to have this sense of ‘duty’ – of wanting to do the proper outward thing, but it certainly wasn’t dedication either.
One afternoon, I wandered into the classroom about an hour early as usual – to get set up and be available for questions – and there sat Jeremy and Jane.  They were in their seats, but had no work out and did not appear to have questions.  Early arrival was not their pattern, and I was surprised, but I didn’t think too much about it.  I greeted them, and went to set up.
Later, I wandered over to chat with them and see if they had questions.  It was then that the reason for their early arrival was explained to me.  One of them – I forget which – told me “We have to leave class early this evening, so we came in early to make up for it.”
I’ll let that sink in for a minute.
For a lot of reasons – decisions I made instantly – I decided it best to accept that at face value.  I now wish I’d have explored more.  Sure enough, they later left class considerably early.
This incident started as a ‘funny student-story’ for my storytelling archives. As time passes, the humor has changed to puzzled intrigue.  In the first place, there was this fascinating sense of ‘duty’.  They actually felt some kind of need to replace the minutes they were going to miss in class by being there in the classroom early.  (Was that only to appear conscientious?)
But of course, that pales alongside the obvious question:  Did they really believe those minutes sitting in the empty classroom actually replaced the experience they might have gotten during the class itself?  I find that so hard to phathom that I continue to wonder if they were ‘pulling my leg’ – all the while with straight faces.  I almost prefer the latter, because to believe they were serious leaves so many (too many) loaded questions unanswered – still.
Should I laugh at this story, or be generally troubled?  So far, it’s been both.  Any thoughts?