Question about subplots_adjust

I'm playing with subplots_adjust, and I'm not comfortable with
parameters wspace and hspace.

Reading the doc, and the code, I've understood that:
- left, bottom, right and top can be considered as a "percentage" of
figure's width and height
- wspace and hspace can be considered as a "percentage" of subplots's
width and height

In other words, left/bottom/right/top and wspace/hspace are not given
in the same "unit": the former is given in "figure unit" whereas the
latter is given in "subplot unit".

I find these convention difficult to use when you try, for example, to have:
wspace = left margin + right margin
hspace = top margin + bottom margin

Here is some Python code to be clear:

left_margin, bottom_margin, right_margin, top_margin = 0.06, 0.06, 0, 0.1

I tried, but it doesn't work as intended because i guessed wspace and
hspace are a ratio of figure's size, but it seems they are a ratio of
subplot's size, aren't they?

Is it a design choice or just a matter of fact? Do you want to keep it
that way or do you agree to change it?

For example, with R (where I came from), figure's margin and subplot's
margin are expressed in the same unit: inches or "lines" (a unit
corresponding to the height of a line... very useful).

Nicolas Grilly