What speed is that chip?
Speed binning seems to be a pretty common technique among processor and graphics chip manufacturers. Basically, all Celeron and TNT2 chips are the same, but some chips run faster than others. Manufacturers test chips, and throw them into different speed "bins" depending on how fast they run. That's why there are fewer Ultra TNT2 chips than normal TNT2 chips. All the normal 125MHz chips are the ones that couldn't cut it at 150MHz… is that really the case?
What happens if yields improve and almost all the TNT2 chips are capable of reaching Ultra speeds, but there isn't enough demand for all the Ultra chips? Simple, you just sell the excess Ultra tested chips as normal TNT2s. Of course, the memory used on those TNT2 boards would effectively limit the overclockability of the card. (Note, not all TNT2 chips are capable of ultra speeds -this is only an example.)
Bin there, done that
Likewise, in addition to the processor core's speed limits, a Pentium III processor would also be restricted by the speed of the external L2 cache. That's why we're sometimes able to overclock a P3 farther when you disable the L2 cache. The Celeron is unique because it already has the L2 cache on-chip, and you don't have to worry about having external cache holding back the core. If a Celeron CPU passes the 500MHz tests, that means the on-chip L2 cache passes the test too.
Now we have to ask how is it that almost all of those new Celeron 366 chips can reach 550? Intel is constantly improving manufacturing processes to improve yields. Whether it's better equipment, materials, or technique, the end result is an increased amount of viable chips from each wafer, production run, or whatever measurement you use to set yields. While demand for Celeron processors still hover around the 433MHz mark, many Celeron chips are capable of reaching much higher speeds.
Let's say that the market demand for Celerons are as follows: 15% for the Celeron 366, 30% for the Celeron 433, 35% for the Celeron 466, and 20% for the Celeron 500. If binning places 70% of the chips in the 500MHz bin, 25% into the 466 bin, and the rest into the 433 bin, there's a good chance that Celeron 366 you buy is going to be able to overclock. While businesses never operate equipment beyond specifications, end-users might enjoy trying to get the full potential out of their Celerons. (Again, this is only an example. We have absolutely no information about Intel's yields.)