It's not brassica oleracea though, it's a different species, brassica rapa. The same species as napa cabbage, brocolli rabe, and bok choy. Rutabaga is actually a hybrid of the two species.
The main reason for there being so many varieties is that at some point in the past the plants genome tripled. With three copies of the genome not only was there a higher chance of mutation but a mutation didn't deleted the original dna segment as it was in the other copies.
They don’t taste like mustard
They're all mustard, so technically they do.
Touchẽ
Sometimes my sprouts and cabbage will taste hot/peppery/horseradish-y.
Damn, xkcd AND botany jokes?? I'm in heaven