I've seen that with some Apple laptops for a long time. I don't know if it is due to the key caps themselves, or perhaps the oils or lotions on people's hands....it may actually be both. The differences may be due to Apple using different manufacturers for the keyboards (Apple tends to source most components from two different vendors).
If the laptop is still under warranty, then have Apple examine it to see if they will address the issue under warranty (doubtful, but doesn't hurt to try). An Apple Store is probably best since AASPs are unable to make exceptions to Apple's official rules. If Apple won't address the issue under warranty or if the laptop is out of warranty, then you can provide Apple with product feedback here (Apple won't respond):
Product Feedback - Apple
You can also contact Apple corporate (Apple may respond):
Contact - How to Contact Us - Apple
You can also check out your local consumer protection laws.