From the FlightExplorer flightpath animations, and statements from the company who produce FlightExplorer, it is clear that the data is NOT predicted flightpath data, but is driven by radar data supplied by the FAA.
The FlightExplorer flightpaths were used by 11 news agencies on 9/11, and the FAA themselves regularly used FlightExplorer flightpath recordings until 1999, at which point they began recording them internally.
The FlightExplorer flightpaths include all deviations from the planned flightpath, confirming they are position based rather than predictive.
The FlightExplorer flightpaths produced by FlightExplorer themselves used their internal data, negating all client refresh timing updates, and so incorporate all FAA supplied positional data.
FlightExplorer received regular requests from the FAA themselves for flightpath traces, confirming the accuracy and validity of their data.
The label for UAL175 at 8:59am specifies the altitude is 31000 feet.
The following image contains the official altitude data for 'Flight 175':
"Flight 175" altitude at 8:59am should have been 22000 feet.
FlightExplorer data validity is clear. Why does the 'official' altitude data differ ? Could it be tracking a different aircraft ?
Here are some quotes from FlightExplorer themselves:
The actual screen refreshes every 10 seconds with any position changes that have been received in the previous 10 second window
How real-time is it?
Sabre Flight Explorer Professional users receive individual aircraft updates every minute. The Sabre Flight Explorer client software that you run from your PC receives updates from our servers every 10 seconds. This means that you receive fresh air traffic data every 10 seconds.
This quote clarifies that the client refresh rate back in 2001 was lower, but does not confirm whether the internal feed rate was any different:
Flight Explorer updates most aircraft positions once every 3 minutes. The actual screen refreshes every 10 seconds with any position changes that have been received in the previous 10 second window.
What type of data is supplied?
Sabre Flight Explorer provides the most accurate and reliable stream of air traffic data available in the industry. The data is based upon radar, satellite and other tracking information supplied by the FAA and other sources. Once Sabre Flight Explorer receives this data, it is processed and sent to the Sabre Flight Explorer client software via the Internet.
Further relevant information on FightExplorer, including the flightpath traces released can be found here:
Flight Explorer Thread