3mm is the diameter of the shaft of the screwdriver, not the circumference - if not, the picture in misleading.
No, I think they are showing the “bit” diameters. Look at the 1.5 mm driver (Y000).
Granted that the shaft diameter can be important too. If it is bigger than the bit diameter, the recess the screw head is in may prevent the bit from ever reaching the screw head - because the shaft hung up in the recess “shaft”. Been there, done that. Among other similar headaches. :-(