gnu/unix sort numerical only using first column?
Sadly you haven't missed anything. This apparently simple task - split lines into fields and then sort numerically on all of them - can't be done by the unix sort program. You just have to figure out how many columns there are and name them all individually as keys.
What's happening when you specify -n no other options is that the whole line is being passed to the "convert string to number" routine, which converts the number at the start of the line and ignores the rest. The split into fields is not done at all.
Your first example, without -n, is also doing whole-line comparison. It's not comparing "a" to "a" then "b" to "c". It's comparing "a b" to "a c".