The idea behind narrowband filters is that they are centered one a specific wavelength and basicly allow only the photons with that wavelength to pass through. Of course, this would be for an ideal filter. Even the best filters have a bandpass of a few nanometers. You may find their bandpass to vary from 2 to 32 nanometers depending on the filter. This is not an issue, is just an engeneering problem, I guess. Anwway, the images given by the Hydrogen alpha filter will be different than the Hydrogen beta filter because they correspond to different emission lines. The H alpha is around 656.28 nanometers and the H beta is around 486.1 nanometers. As to why these to emission lines are different, well this is due to the way bodies emit light. If you aproximate a body to a Black Body than you can say that the flux of light it emits is described by Planck's law for Black Body Radiation. Since stars are a good aproximation to a Black Body we may very well take them as one. Depending on their temperature they emit more light at some wavelenghts that at onthers. Many of them emit far more light in the Hydrogen alpha line than in the Hydrogen beta. For the same time expusore, the Hydrogen alpha image would be brighter the the Hydrogen beta one. That's why they are different
Adrian