It's because English is very, very bad at describing hair color, and something like 75% of us are reading "brunette" to mean "dark" because it does in our English. (One of my fandoms has this fight periodically, where any given person draws the line between "brunette" and "blond" in the middle of "brown" is *wildly* variable, but I think you're drawing it farther to the light side than most people. That said I think LW is too.)