i know this might be hard to understand, but nexus is dead nuts right on.
MS can check all 1024k of the bios - but if you've tied a18 and a19 - THERE'S NOTHING THEY CAN DO IN SOFTWARE TO EXAMINE ANYTHING OTHER THAN THE CURRENT 256K BANK. you've disabled the ability for the xbox to address the other 3 banks, and it will look like 1 1024k image with 4 copies of whatever image you've selected.
xbox live DOES check all 1024k. but if you've performed a hardware mod that let's YOU control the address pins instead of the xbox, when xbox live does it's check it will find 4 copies of the retail BIOS, just like it expects.
before anyone posts saying this is bullsh!t, please take a moment to learn 1st semester digital electronics before making yourself look like a fool.