I can’t speak for the default beats as I think the volume is fairly uniform.
If you are referring to user-created content, there could be several variables at play: the user-created drum set (some seem to be hotter than others); the velocities within the MIDI source file (they vary greatly); and, how users transcribe their content (some don’t adjust the velocities up or down when they work up the song).
There are some forum posts that address this lack of uniform volume. Suggestions have included the use of compressors, mixers, volume foot pedals, and what-have-you. Here’s one that you might find helpful Individual "song" volume setting.
If you’re referring to my songs, sorry 'bout that. I try to attenuate the velocities of the bass guitar (to ~33 or 24, and it depends on the kit I choose to use for the song) as well as the toms and the cymbals which tend to be harsh. I try to stick to just a few kits but that’s not always possible.