serial codes and online games

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I would say its because of the algorithm the key must pass. For online gameing the keys are registered i think but on the cd is only an algorithm which the key must pass. Here is an Example what I mean:

2+6 = 8 //online algorithm but:

6+2 = 8
4+4 = 8
5+3 = 8

Maybe I'm totaly wrong but this are my thougts about the online keys.