I think mechanics change a lot as you get faster… For exmaple a 23.0 sprinter can run a 90% workout several different ways… i.e., they can run a 25.3 smooth and easy with good accel and then chill, or they can be kind lazy in the initial part and really bang the last part… that's why sometimes it's hard to explain what we do because just looking at workouts and times and only part of the equation.
but in general tempo can get sloppy easily — especially when it's overprescribed **or** when athletes think the point is just to finish or hit the times.