The other side is that at speed the circumference of the tire can possibly
grow from centrifugal force. A grossly exaggerated example are the rear
slicks on a Top Fuel dragster. So, the static measurement might just be the
best "averaged" measurement.
Interestingly in Fred Phun's book "How to make your car handle" (page 119 -
at the bottom) it states that manufactures allow up to 7% variations
regarding tire dimensions for a given size (Tire & Rim Association, Inc.).
That 7% is equal to 2:88 gears becoming right about 3.07 gears if you are
changing tires from both extremes of that 7% limit.
By the way, I always thought the 7% factor would be a good argument to get
out of a mildly over the limit speeding ticket. :-)