All manufacturers want to get the most out of the process.
Some silicon has defects in the cache - hence models like the Celeron / Pentium / Duron / Athlon (new) and so on.
Some silicon won't run at higher frequencies ... hence the lower model numbers running at lower speeds (locked).
I think there is also a case that some manufacturing processes mature to the point (Tock) where higher Steppings produce much more viable high end silicon which they have to deliberately laser off cache and lock at a lower speed ... just to target the sales are particular price points.
You will never really know as the topic is taboo for Intel / AMD / NVidia etc ...
But given how many X3's unlock, and how high both Intel and AMD cpu's overclock ... well you make up your own mind.
Lower end parts also sell in greater volume ...
My old E5300 runs at 4Ghz with a push ... its supposed to run at 2.6 ... it has a 2Mb Cache while big brother Wolfies have 6Mb cache.
Who is to say it wasn't a 6Mb Wolfy to start with .. and didn't get neutered for no reason?
Poor little fella.