Reasoning retention factor of benzoic acid
The answer is the dimer formation of the benzoic acid. This is why higher concentrations lead to more extended dimer formation, thus higher $\mathrm{R_f}$ values as the carboxyl group "gets shielded."

One thing of note is that $\mathrm{R_f}$ values are not directly dependent on the polarity. They also depend for example on the size of the molecule (Oswald radius). And the relation between properties of the molecules (as stated for example in the form of molecular descriptors like $\log P$) and the resulting $\mathrm{R_f}$ is very complicated indeed. This is also illustrated by the fact that there are predictors for IR spectra, NMR spectra, solubility and lots of molecular descriptors but I have never heard of a good $\mathrm{R_f}$ value predictor. 

This is why I would always be careful to talk about it like "the polarity" of the compound, especially as "the polarity" is not a clearly defined quantity.