  Radii of nuclear charge distributions carry information about the strong and
electromagnetic forces acting inside the atomic nucleus. While the global
behavior of nuclear charge radii is governed by the bulk properties of nuclear
matter, their local trends are affected by quantum motion of proton and neutron
nuclear constituents. The measured differential charge radii $\delta\langle
r^2_c\rangle$ between neutron numbers $N=28$ and $N=40$ exhibit a universal
pattern as a function of $n=N-28$ that is independent of the atomic number.
Here we analyze this remarkable behavior in even-even nuclei from calcium to
zinc using two state-of-the-art theories based on quantified nuclear
interactions: the ab-initio coupled cluster theory and nuclear density
functional theory. Both theories reproduce the smooth rise of differential
charge radii and their weak dependence on the atomic number. By considering a
large set of isotopic chains, we show that this trend can be captured by just
two parameters: the slope and curvature of ${\delta\langle r^2_c\rangle(n)}$.
We demonstrate that these parameters show appreciable model dependence, and the
statistical analysis indicates that they are not correlated with any single
model property, i.e., they are impacted by both bulk nuclear properties as well
as shell structure.
