Robin,
Waypoints were rarely taken from OS charts. They were nearly always taken from the half mil LFC. If you achieved 0.5nm you were doing quite well.
Now LZ, DZ, TGT, OFFSET, they were different. But a Waypoint / Turning point? I don't see anything sinister or flawed with this degree of accuracy.
Remember, you did not have to overfly your Waypoint. It was just something to attract your attention.