Usually it's just a side-effect of how the system is wired. Consider that GB WRAM mirror, where C000 = E000 all the way up to DDFF = FDFF. Let's look at these

Usually it's just a side-effect of how the system is wired. Consider that GB WRAM mirror, where C000 = E000 all the way up to DDFF = FDFF. Let's look at these numbers in binary: C000 = 1100 0000 0000 0000 E000 = 1110 0000 0000 0000 DDFF = 1101 1101 1111 1111 FDFF = 1111 1101 1111 1111 If you notice, that third most significant bit is the only difference - and this is true the entire way through the mirror. Because that address line wasn't needed to access WRAM, it was probably never even wired to it - so on access the value of it is simply ignored.