An unexpected hierarchy of visual emphasis

As a followup to my “beef it” macro for LaTeX, I’ve found something funny about emphasizing text. Let’s discuss.

When preparing text, there are multiple ways to add emphasis. You can use ALL CAPS ¹. Or you can use italics. Or perhaps try bold. My lovely bfit (bold face-italics) macro stacks bold and italics together, to really beef it up.

You’d think that bold and italics would stack and given even more emphasis:

Emphasis-level 0: Lorem ipsum dolor sit amet
Emphasis-level 1: Lorem ipsum dolor sit amet
Emphasis-level 2: Lorem ipsum dolor sit amet
Emphasis-level 3: Lorem ipsum dolor sit amet

However, when I look at that, I don’t see the stacking. I actually see a weakening, with the emphasis hierarchy looking more like this:

Emphasis-level 0: Lorem ipsum dolor sit amet
Emphasis-level 1: Lorem ipsum dolor sit amet
Emphasis-level 2: Lorem ipsum dolor sit amet
Emphasis-level 3: Lorem ipsum dolor sit amet

I would guess that it $<$ bfit $<$ bf should depend on typefaces, colors, and other formatting details, but it actually seems quite robust to me. I’m not a typography expert, but I do find it fascinating. I wonder what precisely is going on? I suppose it’s just that bold text strokes are narrower after italicizing than before. But even a typeface like Arial ² that doesn’t seem to change the width of bold when italicizing still has bfit $<$ bf. Does it look bad to keep the same horizontal width in the strokes? Do any bold-and-italicized typefaces not have narrower glyphs?

Emphasis-level 99: ALL CAPS ALL THE THINGS.