That'll be a quirk in the programming. GPS satellites are essentially highly accurate clocks combined with radio beacons. They send nothing more than massively accurate time signals. The device receives those signals, compares timings between the two, quickly does a sheet load of maths, and gives you a result. The receiver either receives the signal or it doesn't - its digital in nature, so a weakened signal still means the same thing as a strong one.
People think the the more satellites the unit can 'see'the more accurate it becomes. That's sort of true, but not quite for the reasons people think. No matter how many birds the unit car see it only does the calculations between 2 at a time, then it'll move on to 2 more, etc.
And the final consideration, GPS was only ever intended to define a location on the earth's surface. Speed came along not as a consequence of any changes to GPS, but because receive units were developed that could do continuous multiple calculations, co,pare themnto the previous ones, do the maths to work out the difference and calculate a time. GPS can be used to calculate altitude but is not especially accurate because it lacks a fleet of birds at a second, much higher, altitude. Truly accurate altitude calculations require the unit it be perfectly stationary for long periods of time, such as bolted to concrete blocks like the
Ordnance Survey do. Aircraft that require a super accurate altitude reading get theirs by cross referencing GPS with barometric and radar data to improve the resolution.
The Galieleo system was designed from scratch to have a constellation of birds at a much higher altitude, but unfortunately the system is compromised by financial and political meddling, and is proving unreliable. In a significant number of the unit launched the hyper accurate super cooled clocks have failed, and the units have had to revert to more conventional time keeping units so none of the promise of additional accuracy or functionality is materialising. The US air force were simply doing it better 45 years ago.