behold the magnetic H.

"However, different 1H or 13C nuclei absorb at slightly different frequencies. This difference results from differences in the small internal magnetic field Be for each 1H or 13C nucleus. Again, the Be field results from the circulation of electrons of the molecule. Different 1H or 13C nuclei have different electron density surrounding them and consequently, are shielded from the applied field Bo by Be fields of different magnitude. Remember, differences in (Bo-Be) translate into differences in the frequency needed to cause the nuclear spin transitions.."

http://orgchem.colorado.edu/hndbksupport/nmrtheory/chemshift.html