I found this other topic addressing the same issue:
http://wordpress.org/support/topic/plugin-polylang-browser-detection?replies=2
So, in reality, the plugin behaviour is set more to detect "the last used language " more than "detecting the browser language" (which it does only for the first time).
I think this is not very convenient: one could have switched language for some reason, but still got the browser set in the other language. On the other hand I understand that the idea behind is that someone may have a browser in a given language but if he/she was reading something in that language on your site, than the site will assume that this is your language, right?
Mmm… is there a way to choose the behaviour, or to implement such an option in a feature release?