I find it absurb too, but there are plenty of reasons why:
The phones themselves needed their own selling points. If the Optiumus 2X has both the dual-core processor along with the NOVA display, it'll make the Black pointless. If I recall correctly, the Black has the NOVA display, but does NOT have a dual-core CPU.
Do you want superior display, or a superior processor?
Also, they're going to be on different networks-- basically giving everyone some perks.
It also gives room for upgrades (the 2.0)-- release an Optimus 2X with a enhanced screen. This is similiar to the Galaxy tab that was released without an AMOLED display, while their Galaxy phones are, and the iPad also.
Samsung recently announced they'll be releasing AMOLED Galaxy Tabs.