Sulforaphane is heat labile, so cooking breaks some of it down. Broccoli and cabbage are fairly low in it, while Brussels sprouts and radishes are quite high. Radishes also have high amounts of sulforaphene, a related compound with similar properties. So it might be cooked vs raw, quantity consumed, -phane vs -phane/-phene, or something else entirely.

Only the R-isomer is found in any appreciable amount in nature, so it’s probably not that unless you’re eating research radishes.