Well, it is listed as "clearance/ground."
I see. You are referencing the feed title, not the frequency title. Being an IFR pilot, you know that most class D towers (at least in the US) combine clearance delivery and ground onto one frequency. I mistakenly assumed that you were under the impression that Boston, as a class B airport, did this, too. Sorry 'bout that.
So I am assuming if I choose that feed that both are being monitored. It lists the clearance frequency as 121.65 and ground as 121.9/121.75, and I am assuming that feed monitors them all.
Correct. However, given that there is one scanner monitoring both frequencies, it is possible that you are hearing a long run of busy activity on the ground frequency and that is dominating the feed.
In no way do I mean to undermine Dave's explanation about the PDC, but as a pilot who routinely flies into Boston Logan for Angel Flight missions, I can tell you that Boston CD does contain a lot of clearance issuances and readbacks, both from GA (corporate being the largest percentage here) and commercial airlines.