Some data is transmitted by the aircraft...that can be wrong too if the wrong Hex code is set, but it's the database info that is often so suspect.
The departure and destination airports,type of aircraft, airline name, registration etc are all pulled from databases set up by enthusiasts.
This is how we get a Robin DR.400 2+2 on a BA flight and a Virgin A.340 that was showing as as Super King Air. G-OMYA was appearing as G-BYGA and so on
Here is a page from http://data.flight24.com
This shows Iberia flight IBE3180 has on several days recently, been operated by ex BA Boeing 737-236 AP-BKC that was destroyed in a crash in Pakistan last month

Just don't believe everything this tracker appear to tell you.





