If it makes you feel any better, you were actually almost right. These days the brassica oleracea has several well-known cultivars, including Brussels sprouts, cabbage, cauliflower, broccoli, kale and kohlrabi, all of which come from the same species of plant.
I thought Brussels sprouts were baby cabbages until I was 28 and I finally saw them still attached to the stalk.
If it makes you feel any better, you were actually almost right. These days the brassica oleracea has several well-known cultivars, including Brussels sprouts, cabbage, cauliflower, broccoli, kale and kohlrabi, all of which come from the same species of plant.
Also, relevant xkcd.