In drilling, there are two are two main ways to interpret the rate of penetration (ROP): Instantaneous and Average. Here are good definitions of the two:
Instantaneous ROP is measured over a finite time or distance, while drilling is still in progress. It gives a snapshot perspective of how a particular formation is being drilled or how the drilling system is functioning under specific operational conditions.
Average ROP is measured over the total interval drilled by a respective BHA from trip-in-hole (TIH) to pull-out-of-hole (POOH).
— G. Mensa-Wilmot and Y. Harjadi
A Smoother Plot
Average ROP is important because it’s a key factor to consider when discussing drilling efficiency, plus it produces a cleaner plot than instantaneous ROP.