B747-400 Mach Effect Impact on Minimum Maneuvering Speed Margin

[conch]

The B747-400 FCTM says 'For lighter weights at altitudes up to approximately 10,000 feet, the flaps-up amber band functions just like the flaps-down amber band described above, with the top of the amber band representing full maneuver capability. Due to increasing Mach effects between 10,000 and 20,000 feet, the maneuver capability at the top of the amber band decreases as altitude increases. Above approximately 20,000 feet, the top of the amber band shows the speed that provides 1.3g maneuver capability to low speed buffet'
As the compressibility of the air starts affecting airspeed indication above speeds of around 250kt and Flaps 1 max. speed is 280kt it makes some sense that this problem does not exist for flaps down because you actually never operate in this speed range.
But why is the Mach effect for flaps-up minimum maneuvering speeds only relevant between 10000 and 20000 ft and no more above 20000 ft ?
What is different above 20000 ft ?
Does anyone have an explanation for this ?

[Goldenrivett]

But why is the Mach effect for flaps-up minimum maneuvering speeds only relevant between 10000 and 20000 ft and no more above 20000 ft ?
Since Max ALT for Flaps = FL200, why would you want to be flying any slower than 1.3 g min maneuver speed. You won't be flying around trying to slow down to F1 select speed would you?

[Gysbreght]

But why is the Mach effect for flaps-up minimum maneuvering speeds only relevant between 10000 and 20000 ft and no more above 20000 ft ?
What is different above 20000 ft ?

This is different above 20,000 ft:

Above approximately 20,000 feet, the top of the amber band shows the speed that provides 1.3g maneuver capability to low speed buffet'

This is wrong in your conception:

As the compressibility of the air starts affecting airspeed indication above speeds of around 250kt

The Mach effects are not limited to the airspeed indication. The speed at which the buffet starts (buffet onset) at 1g or 1.3g or whatever, increases with increasing Mach number, and the amber band takes that into account in order to provide 1.3g maneuver margin relative to buffet onset. The AFM or FCOM should contain a chart that shows the buffet onset conditions.