I remember that Quintin Demps ran a lot faster at the combine than Felix Jones. Significantly faster if I remember correctly.
Then, in the second game of Jones' and Demps' rookie season, Felix took a kickoff, broke into the open and started streaking down the left sideline. Demps was about 5 yards behind him and both were running at full speed. As they crossed the 50 yard line, Felix was starting to pull away, and by the time Felix got close to the goal line, it was clear that he was steadily pulling away from Demps.
There were no angles involved here, either. They both were running along the sideline and were running in a straight line towards the end zone, one right behind the other. Felix is just faster on the football field than Demps is, regardless of what their 40 times said. Plus, he was carrying the football and couldn't pump his arms normally as Demps was able to.
While the 40 yard dash at the combine gives a pretty good idea of player's speed... it is just a guideline. Watch people playing football if you want to know how fast they are when playing football.
Jones is flat out fast in pads and he cuts effortlessly. He is one of the faster RB out there, that's for sure.