Let me try to explain the thought process. I'm overtaking a row of middle-lane hoggers at 75mph in lane 3. My speedo says 75 but I know that my true speed will be less as speedos are set to over- rather than under-read your speed. I also know that if I'm actually doing 75 then I am very unlikely to get done for it. Rightly or wrongly, that is what I am thinking.
Just as I am about to pass a particular car in the middle lane, the driver looks in the mirror (or not), sees me travelling faster than they are but decides to pull in front of me anyway, forcing me to slow down. They then proceed to overtake 5 cars at 69.9 mph (according to their speedo, but in reality probably closer to 65) without taking the opportunity to move over to let me pass them until there is at least half a mile of space in the middle lane for them to use.
That's why 'speeders' get frustrated.