If a Palm Bay officer is logged in on PBPD talkgroups and drives to, say, the VIera Courthouse, or the BCSO sheriff's station on Merritt Island or Titusville, as he moves through the various coverage areas, his radio will switch sites, affiliate, and the talkgroup that he is logged in on will now be carried on that site.
Motorola calls this a SmartZone setup. Harris calls it Networked. The system transfers the talkgroup you need to the site you need it at.
If you're actually hearing specific frequencies assigned to Palm Bay in northern areas, it could be either tropoducting or it may be part of the ongoing upgrade process. Eventually the county system will have a north simulcast cell and a south simulcast cell with some overlap in coverage.