....it illustrates, I hope, that the science of legroom measuring is still confounding.QUOTE]
The SAE prescribes how leg room is to be measured. It's simply a straight arc from the hip point to the heel point of a driver of the maximum accommodation size that the vehicle is designed for (typically 90th to 99th percentile male). Therefore you will typically see all vehicles in a similar range for front seat legroom -- it really has nothing to do with how far back the seat track will allow you to move.
If you think about the same size person sitting in an an F150 vs. a Mustang, that arc from hip point to heel point will be longer (therefore "more" legroom) in the Mustang because the driver is closer to the floor and therefore the legs are more stretched out....