They short answer is that they aren't. Exact sizes will, of course depend on manufacturing tolerances but 0.2mm is a lot undersized by normal standards, although for jobber drills this won't actually matter much.
For example metric tap drill sizes are generally specified to at least 0.1mm
One possible explanation for measuring drills as undersized is that the shank often gets worn, especially if it has slipped in a chuck and it is difficult to accurately measure the diameter across the flutes.
The actual size of the hole produced by a given drill is a different matter and will depend on the drilling setup and the material being drilled. Twin flute twist drills tend to produce slightly triangular or hexagonal holes, especially when the material being drilled is significantly thinner than the diameter of the drill, but this is more to do with flexing then the actual diameter of the drill bit. This tends to make them functionally undersized in terms of fitting bolts etc and for general fabrication it is normal practice to go 0.5-1.0mm above nominal size for a clearance hole for a metric bolt (notwithstanding any additional issues with fit and clearance between sets of holes etc).
With twin flute twist drills in general it tends to be the roundness, and in some cases straightness, of holes which is the limiting factor on accuracy. Better tolerances are usually achieved by using a reamer, a multi-flute drill/mill or by boring with a lathe, mill or specialist drilling machine. There are also specialist drills eg gun drills for use where the hole depth is very much larger than the hole diameter.