OK, there is a whole lot of info here, and I don't exactly know how in-depth you want this, but here goes.
When AMD/Intel release a "new" speed of processor, it is not really a new chip. The
core of the processor, which is the layout of transistors on the chip, actually is exactly the same (unless, of course, they release a new core). What ends up happening is they fabricate, for instance, 1000 chips. All these chips are the same core, same exact internal design. They then begin to put these processors through a rigerous testing process. All of the parts of the chip are tested to ensure proper operation. So, in our little example, say 850 of the chips pass this test. The other 150 chips are thrown away. Now, they take the remaining chips and they start to ramp up both the clock multiplier and the ambient tempurature around the chip. As the multiplier/tempurature gradually increases, more and more chips begin to fail. The company, who has done extensive testing on this procedure, knows that when a chip fails at a certain multiplier/tempurature, then that chip can run reliably at a certain speed. That chip is then fixed at the stable multiplier and batched together with like chips.
The reason that specific chips run at different stable speeds than other chips is due to a very large range of reasons. These can range from the overall sterility of the manufacturing environment, tempurature, equipment, etc. Basically, you are creating such small structures on the silicon that there is an inherant margin of error. That margin of error produces minute differences between every single chip.
There is another whole ball of wax (which I will not get into) which involves the actual release of the new speeds of processors, and it is largely a marketing function.
Anyway, so the basic point is that the manufacturer niether "underclocks" nor "overclocks" thier CPUs, they run them at the highest tempurature at which that particular chip will run stable.
The second part of your question is more of a marketing question as well. If they have not produced enough chips at a certain speed to meet the demand, then they will either drop the price on a higher speed chip, or they will make more chips of the popular speed by locking the multipliers of better chips at that lower speed.
I hope all this gibberish helps you in some way
