Currently I'm running 205/50-15 ES100 tires (slightly smaller rolling diameter than stock 195/55). On several occasions while driving on the interstate, I have timed myself between mile markers over a certain distance, and then calculated my actual speed vs. indicated speed.

I have found that my speedometer reads about 4-5 mph fast at highway speeds, so an indicated speed of 75mph is actually around 70-71 mph (~5% error).

However I have also noticed that my odometer is almost exactly accurate. For example, I record my odometer reading at a certain mile marker, then 100 miles later I record again, and I have found that my odometer is off by maybe 0.1-0.2 miles over a distance of 100 miles (~0.1% error).

How is this possible, don't the speedometer and odometer both measure from the speed sensor in the transmission?

I have done both of these tests several times and get the same result every time.