I was just about to use some metric taps for the first time (3 and 4mm) when I realised that the set had two sizes of thread pitch for each diameter.These are 3 x 0.5, 3 x 0.6, 4 x 0.7, 4 x 0.75. The wiki on thread sizes wasn't helpful in explaining this difference, and the difference isn't due to coarse or fine thread sizes
The 3 x 0.5 and 4 x 0.7 seem to be the "correct" sizes, so what are the other thread pitches for?