I'm taking a complete stab in the dark here, but in terms of P25 systems (and all trunk systems, AFAIK), there is a message called a "continuation" or "late entry" message which basically says "radio 123 is still talking on talkgroup 4567 on channel 789". This is for radios who, for whatever reason, didn't get the original channel grant message, because they either weren't listening to the control channel or talkgroup at the time, or were powered off when the grant first came up, or something like that. On some systems (IIRC, Motorola analog is the primary example), the radio ID isn't contained in the continuation message. It just says, "yup, conversation still going on on tg 4567 on channel 789", without telling the world which radio was the last to get a channel grant. In such a situation, radios receiving the continuation message wouldn't get the radio ID, they'd just get the TG ID. This is why if you're parked on a TG, you'd get the radio ID more often, because you'd see the initial channel grants more frequently since you're not off on another TG listening to something else.
Now maybe ProVoice voice frames don't include the radio ID, or maybe continuation messages in EDACS (or EDACS PV) don't have the radio ID. I don't know, as I'm not an EDACS expert. Or maybe Uniden's implementation of reading the continuation messages doesn't 'see' the radio ID.