how do seed prices vary so much?

i know most seeds companies are the same people or buying from the same people and reselling them. just look at some of the pictures they use to show you the buds. but today i was looking around and there is such a deifference in pricing between them it makes me wonder. and how foes one company like www.amsterdammariujuaseeds.com able to give you 20 free seeds with every order and www.nirvana.com gives you reward points that dont count for anything....lol....you need to spend quit a few dollars at nivana fr them to add up.and then theres http://www.marijuana-seeds.nl that has wonderful sales that almost seem to good to be true. i have order from all 3 places and recieved my orders but amsterdamseeds had the best with 20 free mystery seeds. so please if anyone can tell my why they vary so much?

I love the prices at www.marijuana-seeds.nl but I question the legitimacy of the genetics.

Anyway, here is one from that site:

now what is wrong with their genetics. it looks like you have a nice yield going there. i am just starting to grow them out now so i will see how it goes hope mine turn out just as well as yours. they do have some good prices there though. some as cheap as 20 bucks for 10 seeds. even if the genetics are so so still not a bad deal. i just finally got my reoreder from navana and started 3 ak-48 from them today. right now i am vegging super skunk(seedling) acaura, ak-47 bubblishous,northern lights and some mystery seeds from amsterdammarijunaseeds.com. the 20 free they send out. do my best to keep a little variety in my life. cloned an bunch this week and will be moving everyone into flower by the weekend.