I'm probably guilty of it, but there are a few reasons. When you ride the same route everyday
a) it gets boring
b) you learn the light cycles
c) it doesn't seem like you're going that fast

Also as mentioned, it could be down to differences in your bikes or "engine". My 9 mi commute into work on a 700c bike never seems to take more than 40 minutes even when I go slow for dark or wet conditions, but if I have the studded MTB it'll be a good 5 min longer.

And also as mentioned I also commute for exercise/health as well as enjoyment, but not really recreation.

I only ride on the bike path for 1-2mi. The rest of my ride is on the road, so I am mostly keeping up with cars instead of buzzing cyclists...