It could be that the building was required to have voice evac due to it’s size or occupancy. I’ve actually heard of situations where the AHJ inspected a building after horn/strobes had already been installed, but determined it to need voice evac and the horn/strobes had to be ripped out. It could also be that the college switched service companies after the Exceders were installed and decided/was persuaded to do a total upgrade.
More than likely, the alarms on the walls are also speaker/strobes or maybe remote strobes. There are some installations where speaker/strobes will only be installed in large spaces with high occupancy capacities where voice evac is required (such as auditoriums, gyms, and cafeterias) while the rest of the building has horn/strobes. Doesn’t sound like that’s the case at your college though.