The XP version of AMD's CPUs are compared against the 533Mhz CPUs by Intel. For example an AMD XP 2500+ is comparable to Intel's 2.53 533Mhz.
However, when Intel bumped their FSB to 800Mhz, this increased their performance. The same 2.53 would now run much faster.
AMD did the same thing. Their 2200+ processor runs at 1.83. The 2500+ also runs at 1.83. The only difference is the latter has a higher FSB. The 2200 (Thorougbred) runs at 266 fsb. The 2500 (Barton) runs at 333.
One more thing to look at. The AMD XP 3200+ is not nearly as fast as the AMD 64 3200+. They both have the same "3200" rating and they both run at 2.2Ghz, but the former is definitely slower.
Intel does this too. They have a 3.2 Northwood & Prescott cores. Some Northwoods run at 533Mhz and some run at 800Mhz. The Prescott doubles their cache and adds other stuff to the chip. Plus they have some Northwoods that are called Extreme Editions. Mind you, all have the 3.2 rating, but there are so many different versions and their performance all varies so drastically.
In summary, the only reason we have to put up with the ratings is for MARKETING. Afterall, how would you explain AMD getting much more performance out of a slower CPU? People see 4Ghz and wet themselves. If AMD's could provide the same performance, but only running at 1Ghz, they could potentially loose out on a lot of profits from non-tech savy customers.
Thankfully, AMD has started to get away from their rating system and have released a new name for their fastest chip - the FX-51.