They measure the heels from the back of the shoe,
but your heel is not resting there, its actually resting down the incline.
The steeper the incline, the farther down from it the heel is actually resting,
not to mention the foot slipping towards the front due to gravity
and the pressure of walking and deformation of the shoe
from constant toe pressure
on the front of the shoe (that's where bunions come from),
which means the heel of the foot rests even lower.