ffmpeg default output frame rate
r_frame_rate is "the lowest framerate with which all timestamps can be represented accurately (it is the least common multiple of all framerates in the stream)."

avg_frame_rate is just that: total duration / total # of frames

You can just specify -r 30000/1001 to maintain the average rate (near-abouts). You don't specify which format you're outputting to, but for MP4, ffmpeg defaults to constant-frame rate, where it picks r_frame_rate as the value. It will then duplicate or drop frames to keep that rate. Use -vsync vfr to keep the variable rate.