Exercise harder than you think you need to

I just came across this on CNN's Health Section.

A new study done at Canada's York University found that people underestimate how hard they should be hitting it when they're working out.

"Despite being given the standards of intensity defined by Canada’s Physical Activity Guide, volunteers underestimated how hard they should be exercising to reach the “moderate” and “vigorous” intensities. Conversely, participants were accurately able to estimate “light” intensity exercises."

Why do I not find that surprising?