Okay, again, so much clarity in hindsight.
I also see another source of my confusion. Apparently, a stored proc is owned by whoever owns the schema it is in, by default, unless its specifically changed via ALTER AUTHORIZATION. So while schema != owner, the schema owner IS the procedure owner, unless it's been specifically changed.
In my case, I'm seeing sa run these things because the proc is in the dbo schema, and the dbo schema is owned by dbo. I guess dbo = sa when it translates to login.
More info on default ownership