Using Ibuypower confused

[Wampa00]

Okay, when I go to build a desktop on I buy power it takes you to this screen. http://www.ibuypower.com/IbpPages/Intel_X79.aspx

After that I pick which one then start customizing the computer. But if I pick a different base computer then customize it even though all the parts inside are the same the prices vary tremendously.

Can somebody explain why this is?

 

[seller417]

Digital Storm is pricy, but well made.
try here: http://www.gaminggurupc.com/category-s/1819.htm