Because this didn't happen when they transitioned iMac case designs, it didn't happen when they came out with the macbook air case design, it didn't happen when they moved from the TiBook to the PB/MBP case design, and it didn't happen when they moved from iBook clamshell to white iBook, and it didn't happen when they moved from white iBook to Macbook.... so... a hunch i guess?
Apple's projected relatively weaker earnings for the current quarter, citing higher component costs, back-to-school discounts and a "product transition" will cause Apple earnings to come in lower than expected.
Why do you say it did not happen? It would be strange from my pov.
I think the fact that they earned money ALSO introducing new units is different from margin reduction. Every new model, in computer as well as in cars, has the side-effect of reducing margins in the first times. Factories need to change lines, there are design cost, old units must be sold at lower prices...