Also not a biologist and I'm similarly out of my depth, but I'm pretty sure this part of the quoted text is kind of explaining that, but from the perspective of laypeople like us, is kind of glossing over it.

Based on the body surface area of humans and animals, and considering the metabolism and absorption of fluoride in rats

Surface area and mass/volume don't scale the same way (for example the square-cube law- a 1inch cube has a volume of 1 cubic inch, and a surface area of 6 square inches, so a 1:1 ratio of volume to surface area,a 10inch cube has a volume of 1000 cubic inches, and a surface area of only 600 square inches, so a 5:3 ratio of volume to surface area )

I don't know where/how in the body fluoride gets absorbed, but for the sake of argument, let's say it gets absorbed through your stomach lining, so a big limiting factor in how much and how fast you absorb it is how much surface area the inside of your stomach has. More surface area means absorb fluoride more quickly.

So if rats were just scaled-down humans, you'd expect them to need a lower concentration to absorb the same kind of dose as a human.

But rats aren't just scaled down humans. They're rats.

And again, not a biologist, I have basically no idea what the inside of a rat looks like. Maybe their stomachs are roughly the same size proportionally to us, or maybe they're significantly bigger or smaller, which would throw off how much stomach surface area they have available to they absorb fluoride.

And of course their metabolism and body chemistry is going to be different than a human. I'm pretty sure their metabolic rate is way higher than ours so basically everything inside the rat is happening faster, stuff is getting absorbed faster, but also excreted faster, and food/water is spending less time in the stomach leaving less time for that fluoride to get absorbed.

And maybe rats are just fundamentally better or worse at absorbing and metabolizing fluoride than we are, maybe their stomach lining is just more or less capable of absorbing fluoride, maybe they have more or less of some protein or enzyme or something that does something with that fluoride so it gets used more or less efficiently by their body, etc.

So all of that would need to be taken into account. Whole lot of math involved figuring that out that I don't even want to think about.

And, of course, experimentally, we want to be able to see and measure the effects. The study is looking for its effects on the brain, not, for example, liver and kidney function (or whatever organs would be damaged by too much fluoride.) Trying to measure the IQ of a rat I'm sure is already hard enough in general, let alone trying to measure potentially very minute changes in it. It may be they're trying to push the dose as high as they can to try to create any measurable cognitive symptoms, if we're giving the rats 6x the normal dose, maybe to a level where it might damage their kidneys or something, and still not seeing any cognitive issues, it's probably pretty safe to say that a normal, safe, dose isn't going to cause issues either.

Maybe differences in liver or kidney function? I wouldn't be surprised if various organs are proportioned differently compared to humans because of things like volume vs surface area that come with size differences, and in this case diet.