why does the e/m ratio of a proton differ from gas to gas,but whereas not for an electron?

The mass of an electron is 9.1 x 10^-31 kg, which is negligible when compared with the mass of proton, which is 1.67 x 10^-27 kg. Hence, while calculating the mass of an atom, the contribution from the electron is usually ignored. 

Since different gases have different number of protons and neutrons, therefore they have different masses. As a result, when subjected to an electric discharge, the anode rays emitted from each gas had different masses and charges. So, because the anode rays emitted by different gases are distict, therefore the e/m ratio for protons varies from gas to gas.