From what I understand, memory chips that come off the line are tested and then "Binned" according to test results. Chips that don't work are thrown out, chips with low performance are placed in the "Value" bin and chips with high performance are placed in the "Performance" bin. The chips are then assembled into modules, sometimes called "Sticks." Modules with value chips are sold under a value branding with lower specifications (Speed[Mhz], timing, voltage, etc...) and modules with performance chips are sold with a premium price and higher specs.
Also, the kind of chips they use makes a difference as stated by the other posters. I don't think it says on NewEgg the type of chip used in the module; you need to read memory reviews if you want this info.