This has been said before but...
All processor chips come off the same assembly line! (Pretty much, if demand is very high there may be multiple FABs.) There is no "chip headroom". Speed grading is not designed in, it is the result of process control refinement issues in manufacturing. All Bartons were designed to be ~XP3200-3400 parts using a 0.13 micron process.
How it works:
Many silicon wafers (10-12 inch diameter) are cut from each boole of doped silicon. Many wafers are all then simultaneously impregnated by impurities and metal deposited on their surfaces in very large vacuum chambers to form transistors and conductors on the wafers. The wafers are then sliced and diced into individual chips.
All the chips in this batch (and we're talking 10's of thousands) have essentially identical performance characteristics. Small differences come from where on the wafer a die was and where the wafer was sitting in the vacuum chamber.
The die are then checked for gross functionality and the working ones are mounted on substrates. After mounting performance tests are run on each die and the parts are then graded for speed. (The parts that fail the lowest performance ratings get sold to digikey, radio shack and certain undisclosed OEMs.) Now comes the fun part. You get to slap a 300% higher price on the few parts that work the best.
When you first start making a new chip the process isn't perfect so only a few die are the fastest. As you do more and more runs you figure out how to make all the chips come out the same as the best ones and maybe even make them better.
Now comes the sad part. Engineering has now busted their asses for ~12-18 months and done their job. You are making 90% or more of your parts at the highest speed rating possible for the process and design, but marketing has their heads up their butts because they can't find enough fools out there to pay 300% more for a 25% speed increase.
What is a CEO to do????? You can't sell all the chips for the same low price. Marketing advises you "that might wise up the speed freak fools to how stupid they are". So marketing tells engineering to make a substrate with little metal lines that can be burned through with a laser built into the test stand. The little metal lines lie to these strong, bold new chips and tell them that they are slow and stupid. You then sell these large quantities of misinformed chips, along with a few clunkers that were at the back of the line during manufacture, for the usual $100 and keep selling the rest of the identical chips to the fools for $400.
Technical Notes:
1). Intel has more respect for the inteligence of their customers and builds the metal lines into the die itself so they can't be defeated by super glue, conductive paint or pencil lead. You have to push the FSB instead which is a little trickier...
2). As you improve processes you reach the underlying physical limitations of the particular chip design. i.e
a). The capacitance of the transistor junctions and metal runs requires so much current to charge them that more heat goes into the die than can be removed using reasonable cooling methods. (Why the original Tbred & Tbred A were rapidly replaced by the Tbred B dies.)
b). The time it takes the electric fields to move through some parts of the die are longer than it takes to move through other parts of the die and a race condition is created. The die starts doing things out of sequnce, gets confused and stops working.
(Why intel is kicking AMD butt and AMD has to go to 64 bit data paths to keep up. Also why cryo cooling can push a given chip design only so far.)
3). The "pretty much all from the same line" applies to using multiple production lines for die. This used to be common but with larger diameter wafers, smaller die, faster new product introductions it is becoming "not cost effective" to spend all the money needed to rediscover all the tricky little fabrication details that make a perfect part every time on adifferent production line.
Ok, I vented. I feel much better now.
Edit - Mommy read this and said I can't skip my medication anymore...