Why does T-Mobile want to build an LTE network using frequencies Verizon hasn’t touched in six years? There are several reasons, but the main one has to do with the different stages the two carriers are at in their LTE rollouts.

T-Mobile(s tmus) just agreed to pay $2.635 billion for Verizon Wireless’s neglected 700 MHz airwaves, and to boot T-Mobile kicked in some valuable higher-band spectrum it originally planned to use for LTE and HSPA+. According to BTIG analyst Walter Piecyk, the package deal amounts to a 38 percent premium over what Verizon originally paid at auction for the spectrum in 2008.

That’s a lot of money for spectrum that Verizon Wireless no longer wanted and that much of the wireless industry had written off as sub-par airwaves. But the value of spectrum to any given carrier is all in how you look at the network.

When Verizon first gained its…