The pens are sort of "self-adjusted" in that they deliver the correct number of units based on the dial.
If you dial 24 units on the U200 pen, you get 24 units.
If you dial 24 units on the U100 pen, you get 24 units.
With U200, you only need half the volume of insulin as with U100 for the same number of units, hence the ability of the U200 pen to provide about twice the units of insulin as the U100 pen, over the life of the pen.
The mechanical "clicking" and delivery mechanism for both pens in the same, hence the ability of the U100 pen to provide any integer number of units (one "click" equals one unit), whereas the U200 pen does even-numbered doses only (one click equals two units).
If you stick to the numbers on the pen dial, you'll get the units of insulin desired with either pen.