From what I recall the hit counter keeps track of logged PTTs. A counter showing zero means the radio has been observed affiliating to the group, but has never transmitted.
EDIT: Oh wait, sorry... I misread that. So the group shows radios that have lots of hits, but the group hit counter itself is zero. Yeah that's a little odd. I'm not seeing that on any of my setups here.
Hm, I think that's exactly what it would look like when the coppers jump onto a little-used tactical group prior to a raid or other operation. Reminds me of what TRUNK88 was showing at 0330 on June 13th, 2013 as "Project Traveller" spun up. Hundreds of radios powered up and affiliated to ten or so infrequently-used talkgroups. When that happens, you know the hammer is about to drop. That was a wild morning.