Trunking systems are a bit trickier and even more reason to change your range on the scanner down to zero (the range after you tap the 'range' icon on the screen, not the 'radius' in a system in the favorites list).
Professional trunked radios listen to the data a trunk site sends out and only affiliate/listen if the site number matches the site they plan to be on. (That's really simplified again, but it serves the point.) However, multiple sites around the state can have the same frequencies. For example, the WISCOM site in Jackson County is Black River Falls, and according to the RRDB, the control channel is on 139.1125. However, that frequency is also on the Milton site in Rock County, and on the Squirrel Hill site in Oneida County, and Iron River in Bayfield County, and a bunch of others. The HP1 (and generally all scanners) don't bother checking to see that the site broadcasting on 139.1125 is actually site 020-020 (Black River Falls), it just checks to make sure that the system ID is correct, and then lets you hear whatever is going on (based on your programming). So, if you have your range set far enough so that one of these other sites using the same frequency is "unlocked", the scanner could be telling you that you're picking up the more distant site, even though you're not.
As for the actual transmissions from various agencies, trunk systems tend to transmit traffic to a site only if there is a radio registered (or "affiliated") to that site to hear them. So if nobody from Eau Claire DOJ, for example, is on the Black River Falls tower, you won't hear anything from that talkgroup on that tower, even if it's really busy on the tower closest to Eau Claire. The exception to this is, system designers can configure a talkgroup to be present on all (or a subset of "all") towers. That's probably what the "travel" talkgroups on WISCOM are for - they are probably configured to always broadcast on their home area's tower, plus whereever radios are affiliated.
Hope this doesn't confuse things too much. As always the wiki will have some good info if you want to know more.