Why can't soms cpu's not be oc so much?

[gasolin]

I have a Asrock H55M LE and a i7 870 cpu, and to me it sounds like something i could oc to 4.0 ghz, but i can't, if i want to be able to take 10 passes with intelburntest on maximum and pass the test.

Why is there some cpu's that can't be oc more then a few % i only have my overclock mode on optimised that give me only a few % of oc,and my ram at 800-1600mhz (running 803mhz) now my std ghz is just under 3100mhz, i get 11 in superpi, my oc'ed Q6600 from 2.4 to 3.0 ghz (that oc was extreme easy) got about 18 in Superpi, so i do have a much faster cpu now

I am using a 620watt 80+ seasonic psu, 2x4gb of 1600 mhz kingston hyper x ddr 3 ram, 5 fractal cabinet fans and a cooler master v8 (cpu cooler) is a H55 mb that bad?

 

[Z1NONLY]

There are a lot of variables involved in overclocking but regarding just the CPU's:

The manufacturing process doesn't produce chips with 100% identical properties.


Oversimplified explanation:

They are tested to meet certain specs. Some pass with flying colors, other just make the cut.

If you get a chip that just made the cut, you probably won't be able to go far past the stock specs.