H&FJ's (as it was at the time) Mercury was designed to keep the same spacing in different weights to handle the physical needs of printing a newspaper in different locations and conditions, see the design notes on the typeface [1]. That's the first time I remember this quality being touted in a font (among their set published weights, you couldn't vary it minutely, say, to get a bold-semibold).
While you can try to adjust tracking to make one weight of a face match the spacing of another, the faces are probably individually adjusted to make their own default spacing coherent within the face. It may be different enough between just two faces to sometimes not work, and even less across the entire range.
Adobe Acrobat used a version of Myriad and Minion as under-the-hood variable font substitution to make every PDF readable at least even if you didn't have the original typeface. The promise was you would never see Courier again. Multiple Master fonts made available this idea where you could generate specific typefaces along different axes for your design needs, but it wasn't a live thing, and you ended up with lots of discrete font files. My recollection of Multiple Master was that everyone hated them and were glad to see them die. Maybe the automated adjusting of parameters makes the implementation palatable now.