I'm wondering if this isn't just a case of more bottled water (i.e. less consumption of fluoridated tap water) and more educational resources being available to the more affluent.
Discussion
Even if you don't drink it, if you don't have a whole-house water filter, you're getting it through your skin when you take a shower.
It will also depends on if your local government is adding it to your water supply. For example in the UK, Scotland and Wales do not fluoridate their water.
I used to drink mostly bottled water in glass, but once I started digging into those suppliers, I invested in filters and drink only that.
Also, I would say it is important to pay attention to your dental products, since they mostly add it under the BS claim of it helping your teeth.
Fair enough. However, none of that is inconsistent with the hypothesis I proposed. Absorbing fluoridated water through the skin via taking showers is still consuming less fluoride than if you are also drinking it.
Of course, it is *only* a hypothesis that *could* explain the results presented - at least directionally.