This could come from the presumption that most people won't have set that preference, so it would be correct more often to assume language based on IP? I mean I doubt it, since even the setting is automatic based on where you download your browser in most cases, but whatevs, let's be generous in _our_ assumptions.
It doesn't make it any less annoying though. What tips it over the edge into maddening, is not allowing me to correct the assumption, or overriding my choice when I do.