The data feed for FR24 comes from multiple ADSB receivers in more or less real time but uncoordinated. At periods of heavy load there can be a slight lag decoding and validating them and assigning to an aircraft/flight number, or there may be an uncovered location. It's not a hard real time display, it's near real time. They do also extrapolate position based on history and sometimes get that wrong slightly, it's not unusual to see a trace of an arriving aircraft shooting off the end of a runway for example, and then correcting a little later when a good transponder signal turns up. So some incorrect positions do sometimes sneak in.