All big vendors will place orders some distance into the future. Lenovo does it, too.

You can't lock in prices forever, though. The more volume and stability you have, the more a vendor will be willing to enter long-term agreements with you. Lenovo has less volume than Apple and is not in as great of a financial position, so they don't have as much leverage.

The bigger factor is that Apple already had more margin in their products. The price premium for RAM upgrades on Apple laptops is large, as everyone knows. They could absorb more RAM price increases without being forced to raise retail prices.