The key of this question is the way the drag changes with speed and the maximum thrust the engine can develop at very high altitude, please read carefully the answer, it is a bit complicated, but I am confident
Let us first of all understand what is Vimd (Indicated minimum drag).
Said simply it is the speed of minimum drag, but let us understand a bit more
Drag has two components: induced drag and parasitic drag
- induced drag is directly related to lift production and is greatest at low speeds and high angle of attack
- parasitic drag increases in proportion to the square of the aircraft speed.
At any speed total drag is the sum of the above two components
For obvious economical consideration it is desired to fly with the lowest total drag which is also compatible with the desire of not using oversized engines
The following curve gives the graph of total drag versus speed
It appears that Vimd corresponds to the intersection point of the induced drag curve and parasitic drag curve
To answer the question
at low altitude let’s say your stalling speed is around 140K IAS and at cruise level stalling speed is approximately 220K IAS, Why is that?
For better understanding let us consider straightforward what happens at very high altitude.
When flying at high altitude, that is at an altitude where the engines thrust is limited by the low air density, and the pilot being therefore flying very closely to Vimd, if for a reason or another he reduces his speed, or his speed decreases, though still above the stalling speed, his actual engines thrust will rapidly become insufficient to overcome the increasing total drag increased by the speed reduction, as visible on the above curve. To maintain altitude he has to pull on the stick,( thus increasing the AOA and the drag), and simultaneously increase the thrust rapidly; if the thrust is not adequately increased, his speed will rapidly decrease and he might suddenly hit the real stalling speed. To avoid the above scenario the barber pole minimum speed is voluntarily well above the stalling speed, to increase the pilots reactivity.
Normally at high altitude the autopilot and the auto throttle are engaged happily safely they look for the barber pole speed limit and not for the theoretical stalling speed.
the higher the altitude the larger the gap between the barber pole minimum speed and the theoretical stalling speed
Note, June 22, 2019. slightly edited for better comprehension.