Biggy,

Because they are trying to squeeze 3 sub channels out of their alloted bandwidth, one or all of them are going to have to suffer. Broadcasters will typically keep the prime content on the x-1 channel and will use 720 on the main x-1 channel, then 480 on x-2 and x-3 sub channels, however in order to keep all channels working reliably within the alloted bandwidth the x-2, x-3 channels will need higher compression and generally picture quality will suffer more. It seems that x-3 is ussually the sub channel that get's compressed the most since content here is generally of lower priority to the broadcaster. This of course is entirely up to the broadcaster, but in my observations so far it seems that they put greater priority on the first sub channels and the rest is merely extras that they use to make the most revenue out of their alloted channel bandwidth.