背、脊、癶。

Another confluence of kanji: the se that means "the back" in "se ga takai/hikui" (back is high/tall/short, meaning a person is tall/short) is not the same se that's in the vertebrae. 背 (se/sei)　vs　脊 (se/sei), same meaning. Is there a difference in the kanji usage? Per Jisho they're both among the common-use kanji taught in schools and both carry meanings of height/stature, but 背 seems to have a broader range of meanings, including defying/disobeying/rebelling and back/behind. 背 seems pretty straightforward in structure: 月 for flesh, and above it 北 kita north, which is two people sitting back to back. So, the back of the body, and people turning their back on each other. Henshall doesn't seem to have an entry for 脊, and I'm not sure what the upper part of it is. Jisho says, not quite convincingly, that it's the mysterious "dotted tent radical" that we see in 発 hatsu (to discharge) and 登 noboru (to climb). Henshall says that radical is obscure but used to be written as two feet. So, I guess somehow flesh and two feet make height/stature. OK by me.

I'm not so sure, though, that the upper radical in 脊 is 癶. I guess maybe, but they're written very differently; 癶 is hooked left flow, dot, then swipe, right flow, dot; 脊 is written as standard hitoyane (person-roof)—left flow, right flow—and then two dots on the left and two on the right.

See 脊 on Jisho
See 癶 on Jisho

If it is 癶, why is it written so differently in this case? Do any other kanji share this format?

Saiga-jp turns up seven kanji in all with the radical 癶:

登 noboru, to climb (obscure, originally written as a pair of feet, a food vessel, and a pair of hands)

発 discharge, emit, radiate (which in its earlier forms was about standing firm and shooting an arrow)

廃 discard, scrap, become obsolete (hatsu, leaving, in a building) (another phonetic confluence: 廃る sutaru/sutaeru, 捨てる suteru, both to abandon)

澄 clear, limpid, lucid　（noboru with water)

葵 a hollyhock (あおい) (hatsu under grass)

橙 a bitter orange (橙色) (noboru with tree radical)

燈 a light (灯) (noboru with fire)

So, everything with that radical, at least from the dictionary's POV, derives from either hatsu or noboru. 脊 is not among these, and in none of them is 癶 written as it is in 脊.

Mysteriouser and mysteriouser.

Then again, maybe it's really just 个 (counter) with 二 (two, parallel lines) stacked on each side, to suggest a very tall stack of vertebrae. I'm fine with that, but it ain't no 癶.