Thoughts:
Not using the correct speed for take-off is probably due to lack of familiarity with the flight manual figure for the aircraft, lack of recent flying with an instructor, and lack of self briefing regarding the climbout required (is a climb at Vx necessary?)
Not using the correct speed for climbing is probably due to nothing more than a preference for cruise-climb regimes rather than best rate (better vis, engine cooling etc.)
Not using the correct speed for approach is likely to be due to lack of flying with an instructor, but might also be worries about keeping the speed up in a busy circuit.
Not using the correct speed for landing could be due to having trained on long runways. If you learn at Netherthorpe, you are unlikely to land long on a regular basis!
But ultimately, any deficiencies come down to either training or lack of discipline.
I enjoy the fact that I can always improve my flying, and aim to learn and improve on every flight.