Well.. perhaps it doesn't recognize that the disk is a 4X disk, so therefore it wouldn't know that it's compatible at 2X. For all it knows, the disc could be a 1X disc only.
I don't have a Mac, but doing a quick search on "Apple Superdrive Firmware", I found this page:
http://www.apple.com/hardware/superdrive/
Do you have that update installed?