I read that wiki page, but there’s also this post, which is what I was referring to.
There’s another document that says it’s based on source/destination IP pair, but it’s from 2007 so it’s surely outdated.
I couldn’t find a definitive answer.
Just to be sure I tried repeating the test and making sure that the source interface is always the same (I haven’t configured anything relative to ToS or routing marks, so I guess that those parameters should stay the same too in the default configuration), but no luck, problem’s still there.