Yes, they do (although a fair number are usually not working properly, making that bus "untrackable".)
But, for example, the MK bus makes 3 stops at POR before arriving at South Depot. A delay due to wheelchairs or buses stacked up can occur at any or more than one of those stops, causing the ETA to change.
A bus could also fill up before reaching South, causing the ETA to jump back 10 or 15 minutes. If a bus fills up, and everything is working the way it should, another bus should be dispatched from the buses in reserve at the resort to immediately pick up the slack -- but everything doesn't always work the way it should.
And if the next bus displayed on the monitor has not yet left the park, a bus may not be available for that run until a few minutes after scheduled, or the bus assigned may be delayed at the park due to heavy unloading.