I use the Sega musics on mine, which is a fixed pattern (ie: level 1 has music 1, level 2 has music 2) as opposed to the PC version where it just does them in order, advancing each level, and I've always tried to have levels suit the music playing in them, so that's part of the reason for the gaps. There's also, to a lesser degree, that the harder ones generally appear later in their difficulty ranking (but to mimic the style of the original games, there are exceptions - for example, although it's quite a hard level, No Salvation is one of the easiest PSYCHO levels yet is the 2nd to last level in PSYCHO, and on the other hand, Backdraft is one of the harder Danger levels yet appears quite close to the start).