I've made several posts to comp.sys.mac.hardware.misc with the bulk of this info and as new machines come out, and new concerns surface, I will try to add them to this history of the modifications, and post them on comp.sys.mac.hardware.misc. I've also started a Clock Chipping Home Page which has the latest history file, and several other useful files.
All computers operate at a certain frequency with which operations are performed.
The designer of the computer, Apple in this case, will use components that are rated at the same frequency or faster than the final computer will be. In this case the 68030's are made by Motorola. All 68030's are generally alike in what they do, but they are not alike in how fast they can do it. A large frequency difference will require a different mask during production of the processor, but small changes may not. Motorola only needs to guarantee that the chip they mark as 20MHz will function properly at 20MHz under a variety of conditions.