If you searched for only a route and not a vehicle:
Your query will include all GPS data from vehicles that served the selected route during the specified time/date range, even the data that was submitted before and after serving the chosen route. After turning on the GPS Data filter, the user can see all GPS data submitted by every vehicle that traveled along a given route, thereby revealing the path of vehicles before, during, and after serving the selected route.
In this case, the map shows all GPS data during the selected timeframe from every vehicle that was assigned to the E2 route at some point between 3:30AM and 8:30 PM. This includes many data points surrounding the E2 stops (denoted in purple), but also many locations extending southward that are not associated with travel on this route. Upon clicking on one of these points, we can see that this vehicle was serving route (S6) when creating these pings, but is still included in the query because it served route E2 during the selected timeframe of 3:30AM-8:30PM.
Including this data from outside the selected route helps to trace vehicle activity and see where vehicles were before and after serving the selected route. For example, this could help investigate whether a vehicle was already behind schedule before switching to the selected route.
If you are interested in seeing only data points associated with travel on your selected route, you can tighten your time range to exclude instances in which the relevant vehicles were serving other routes. In general, it is best practice to keep GPS Playback queries as concise as possible, and only to search for the exact time and date range that is needed.