Average time is constantly off

I use the app for kayaking. The other day I paddled 6.915 miles in 1.35 hours. The app told me I had an average speed of 4.36 miles per hour. Simple math tells me I averaged 5.1 mph. The app never recorded a speed higher than 5.02 I went back through the past 400 miles of kayaking and have similar discrepancies for all trips. What gives?

Hi. Please mail me STX file for that activity at caynax@gmail.com