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Basically the modem is considered a seperate connection, so it gets its own ip.
think of it as if you have 2 network cards, 1 going to router a and the other going to router b. Router a assigns the card 1 ip A, router b assigns it's card ip B. Now each card needs it's own ip to operate, so your computer has 2 ips, ip A (for network card A), and ip B (for network card B).
This is basically what you are doing (yes, a modem connection will be "seen" as if it was a separate network card (in the above example). This is a bit confusing, but if you need clarification, i'm sure that i can try again, or someone might have an easier way to explain it.
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