The head size is stamped on the die, and that's the number they're supposed to put on the tin.
They don't actually measure the head size, just take whatever the die manufacturer have stamped on the die and put that on the tin.
(That info is from Mr JSB himself, when RobF asked him that question in Germany in 2013)
It's anyone's guess as to why the same die number can apparently have 2 different head sizes.
Labelling error? There's been a replacement die 34 ?