@Australopithecus Correct, however the underlying data will be precise, the web site smooths out the flight paths & attempts to predict where a plane is until it gets an update...often aircraft seem to overshoot the runway, only to latter after an update to their location have their position & track corrected.
However once in cruise...with constant airspeed & heading, the depicted map position will be quite accurate. The location, airspeed, heading details for an aircraft are the actual details from the aircraft. You will notice the aircraft moves but these details do not update. The website is calculating where the plane is to give the impression of more regular updates, but it doesn't change the numerical details withe these calculations. It only updates those with the raw (true) data from the plane itself.