Lets look at a graphic of aircraft speeds. The following is a map illustrating flights following the same ping-rings with different airspeeds. (For illustration only, not drawn to a particular speed-scale.)
Red is the median TAS.
Yellow is faster TAS.
Purple is slower TAS.
(disregarding wind, as it will be te same in each case.)
Each hourly checkpoint is set to the same ping-ring location. And remember that the ping-ring location is a known fixed point (a location on an arc). For instance, if the aircraft ended up on the 40 degree ping-ring after 6 hours, then that is where it must have been - somewhere on that arc. Same for all the previous ping-ring locations at the previous hourly pings (that we still have not been given).
But notice how the faster TAS speed-track trends westerly, while the slower TAS speed-track trends easterly. So if they are looking at a new track (purple) to the northeast of the original (red) track, then they must be looking at slower aircraft speeds, not faster speeds.
I presume what they meant was that the aircraft was flying lower, so it had a faster IAS but a slower TAS.
ie:
39,000' 260 kts IAS = 463 kts TAS. (slower speed = faster TAS)
10,000' 290 kts IAS = 335 kts TAS. (faster speed = slower TAS)
((Remember that if the aircraft flew for 6 and a bit hours, then it flew for 6 and a bit hours - so the fuel burn is irelevant. The aircraft flew until 08:11, whatever the fuel burn was. So if it flew faster it went further. UNLESS, of course, they were referring to IAS rather than TAS.))
