That's because algorithms can't pick up the nuances of what distinguishes, say, Noise Rock and Noise Rock/Shoegaze. You need humans who understand these things.
I've run into this as well. You try to make a station that plays piano rock, and Pandora goes "Hey, here's a song that fits! It has: 1) mixed acoustic and electric instrumentation, 2) major key tonality, and 3) mellow rock instrumentation." It won't care that every song you've added to the station was tagged with acoustic rhythm piano, all tags are equal to the matching algorithm.