MNTA: I'm sure it does, to a smaller degree than one might expect. Please allow the author of that calculation (Dew) to decide how to address it.
For my part, I don't expect it matters much. The difference between a 25% discount and 14% probably isn't linear.
Also, we are speaking of volume purchases, where any discount is material to the buyer.
"If we don't succeed, we run the risk of failure."
-Dan Quayle