The 4.86 years of experience is the mean of the 677 respondents who indicated student.

Going back to school after a few years is one thing... (and I lack the median, but I'm going to guess that it isn't too far from the mean) but having most students go back to school after 4 years of professional experience seems a bit odd.

I really suspect that they're either counting their 2 months of summer internship as a year several times over, misreading the professional part of it and including hobby, or misreading the professional and considering that they're a senior with 3 years of academic experience which translates 1:1 with professional, so they'll put that down.

The issue is that type of "something is fishy here" without further drilling into it makes me more skeptical of other data outliers and suspect more reporting and analysis issues than a (self reported) poll can be trusted.