@DozyWanabee
The eye is much more sensitive to a variation of angle than to a linear movement. Experience that with an ordinary digital photo camera watching clouds passing by in the wind. Moving clouds is very visible to the naked eye following clouds angularly - also particularly close to the line of sight - but on your screen your image becomes linear clouds seem to have stopped.
If indeed the rate of descent of the scale was equal to that of the aircraft we should not be able to read altitude, I remember a 50m /s freefall near a cliff, believe me, it scrolls very very fast! Impossible to read altitude on that Cliff/virtual screen !
In the HUD KLOPFSTEIN emphasized the need to have the same angular distances threw the HUD and threw the windshield. You don't have that with the bird, or worse thé blue digits emerging above or under the yellow figures.