Oskar Fischinger

I don't have an instrument handy to check the notes, but depending on what dots you click I think you can get 2nds, 7ths, and 9ths so technically dissonances are possible. Some notes are 'missing' in that it isn't a full scale so some harsh dissonances will be avoided. It's similar to a pentatonic scale where all the notes sound nice together, because the palette of notes is limited.

If you select all the dots on either a single horizontal or vertical line you will hear one giant, banging chord or one single note repeated. Then it doesn't sound so pretty. It relies on the space between the notes that remain when the typical person just clicks around the grid randomly.

Another factor is the tones of the instruments. They are really great sounds! If you replaced them with low quality sounds, it probably wouldn't sound as impressive.

I hope I don't sound like I am making light of this app. I think it's really impressive. I'm only making a few points about how I think is works musically.

You may be interested in 'musical dice' games. It's another way of making quasi-random music where seemingly any random selection of music works.