You and I may not be capable of manually measuring a pellet's diameter, but trust me... manufacturing and automated tolerance control has gotten to a point where a proper plant can actually do it without having to resort to black magic. In camera manufacturing (and doubtlessly a lot of other branches of manufacturing as well), tolerances are expressed in single micrometers, so 10 micrometers shouldn't really be a problem.
I actually can. About 5 years ago I got a block made with 4.53 - 4.48 holes put in it. Offer up 90% of pellets from any JSB tin and their heads will slip into the 4.50 hole. The tails are too random in size and shape to measure.
That's why I know head size as stated on the tin is just a sticker. As Simon says its the batch that counts.