Cruciferous transmogrification

Last fall, in a desperate attempt to produce Brussels sprouts in my garden, I put in 18 plants. They were a mix of some I had started from seed and a pack of plants purchased from a local garden center. Somehow, out of 18 plants, only 5 are still Brussels sprouts. The remaining plants have been transformed into broccoli. I've heard of fish that can change gender if there aren't enough of both sexes for reproductive purposes, but I've never heard of plants switching cultivar groups within a species.

With the miraculous conversion, the aphid attack which has stunted my Brussels sprouts plants, and the 9 other actual broccoli plants in the garden, the forecast is once again looking bleak on homegrown sprouts. But I sure do have a lotta broccoli...