The results of my magnetic flux data from voltage recorded
There may be some detuning effects, but another effect is that you have a large driving loop. The magnetic field along the axis of a circular loop of wire is actually proportional to
$$B_z \sim \frac{1}{(z^2 + R^2)^{3/2}}$$
where z is the distance along the axis and R is the radius of the loop(reference).
This alone won't account for your entire observation, as it is still steeper than the \$1/z\$ dependence you are seeing, but it is probably part of the story of what is going on.