Why should the proton nuclei in different compounds behave differently in the NMR experiment ?
Because the different nuclei in different compounds are in different chemical environments.
If the protons (or the nuclei) are in different chemical environments, they are necessarily in different magnetic environments. They should respond (and do) respond differently in the NMR experiment.
I can think of 1 example where different nuclei give rise to the the same chemical shift. If your compound has an active hydrogen (i.e. an alcoholic proton or an enolizable proton), it could be replaced by the deuterium isotope,