Like everyone else said. This is where JEDEC comes in.
Its basically an industry standard so that any RAM you purchase for your motherboard will load on boot-up. This allows you to adjust your timings manually after the PC has POSTed. Not every motherboard can run 1066RAM and more importantly many cannot run the higher RAM voltages to reach these speeds. If it were not for these standards you wouldn't even be able to boot the PC when you got new RAM for it. It is easier for the RAM companies to abide by these standards than to program their products to work in every version of every motherboard in existence at the rated specs.
For example, my Crucials have 2 sets of JEDEC specs (1)667MHz and (2)800MHz. My motherboard loads the 800 at first boot, but I imagine if the mobo didnt support 800 it could go to the 667 timings instead.
No offense to the OP, but this is my biggest complaint about people who review RAM on Newegg. They all say the RAM is junk and wont post at rated speeds so they assume its junk and want Newegg to replace.