40 mil units over the lifetime of a product, a 2 year period, vs 40 mil units in one quarter, is a little different. Not as much as a couple of thousand vs 40 mil in a quarter, of course. But as others have mentioned, OLEDs aren't known for color accuracy when it first came out.
Apple screens _are_ known for color accuracy. Until OLEDs can match the color accuracy, Apple wouldn't move to them.