said by ajburgh :

Most current cable companies are still sending out analog channels 1-99, plus a digital copy of it that your cable box is showing instead (the cable box just pretends they are on channels 1-99). These analog channels are what's shown when you hook up your TV directly to the cable.

They take up a lot of space, about 6 SD digital channels or 1-2 HD channels per analog channel. If they stop airing the digital copy like they did in Chicago recently there will be a lot more capacity to add additional channels. However, it also means you can no longer just hook up a regular TV directly to the cable and watch TV like that.

Big cable systems, like the one in Boston, are mostly made up of smaller cable systems that have been taken over in the past. Some of these parts may have a slightly smaller capacity than others. My guess is that Arlington/Somerville has a cable network with a slightly smaller capacity that does not allow them to add the additional HD channels right now. When they clear out all the analog channels there should be plenty of room to add those.

Of course, the only official answer would have to be given by Jason. This is just my understanding of it.