Haddy - you must be buying retail CPUs.
As far as steppings - the entire range of speed grades are available in the same steppings in this case. They all come from the same wafer. Then they are sorted into speed grades - to a point.
It usually works like this with ICs:
Market demand is such that they think they will sell (these figures are just examples - I don't know what AMD's current market mix is)
5% XP2100s
10% XP2000s
15% XP1900s
20% XP1800s
20% XP1700s
30% XP1600s
Then, they look at their current process and see that say all the die will reach at least XP1800 speeds (slightly above mid-point once a process is stable is the usual minimum yield - at later points in the life of a design, this shifts upward towards the 80 percentile point or higher - I suspect they are close to that point now).
So what they do is take a wafer and begin testing them and label them according to how fast they run. Once they have found 5% that meet XP2100 speeds, they shift their test limit to XP2000, once they have 10% that meet that point, they shift to XP1900s. Once they fill that quota and knowing the rest should be in at least the XP1800 range, they simply mark the rest of the processors according to the percentages they need and go to functional testing at the expected yield frequency. Using the example figures above, this would mean about 55% of the die would be simply labelled according to needs without being speed screened at a higher speed.
Out of that 55%, a decent percent even though marked XP1600 will actually reach the same speeds as the XP2100 marked die.
The better they get the process down, the larger percentage will reach a higher speed - having nothing to do with how they are marked.
A way to get an idea at what points on the curve they are yielding, you can usually look at pricing on the OEM parts. You'll notice a tight clump at the bottom with only say a 10% delta in pricing. Then as the yield at a certain speed grade goes down, you will start seeing a rapid ramp up on the price.
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