How pointer to member know which member it bind?

struct Sample {
    double value1;
    double value2;
    double value3;
};

double Mean(std::vector<Sample>::const_iterator begin, 
    std::vector<Sample>::const_iterator end,
    double Sample::* var)
{
    float mean = 0;
    int samples = 0;
    for(; begin != end; begin++) {
        const Sample& s = *begin;
        mean += s.*var;
        samples++;
    }
    mean /= samples;
    return mean;
}

...
double mean = Mean(samples.begin(), samples.end(), &Sample::value2);

code copy from C++: Pointer to class data member “::*”

I think this is a good example to show when and how to use pointer to member, but after go to deeper, I find type of var is double Sample::*.

So my question is how this pointer (var) know it bind to value2 not value1 or value3 just because you pass &Sample::value2?

My guess: I think the &Sample::value2 must be special that to indicate pointer var should point to address offset two double. But key point is how to indicate? I have not found any additional information in clang AST. Please help me. Thx.