Why do some overseas Chinese restaurants translate broccoli as "芥蘭"? I have seen 芥蘭 listed in overseas Chinese restaurants several times as a translation of "broccoli".

This has generally struck me as odd, as I have always referred to broccoli, at least in Cantonese, as 西蘭花. (Wikipedia suggests that this is also common usage in Mandarin.) How common is the first usage? Is this usage possibly linked to regional variation, and if so, which ones in particular?

For what it is worth, I have seen 芥蘭 referred to as "Chinese broccoli". Moreover, I ran Google searches for exact matches (using quotation marks) for all of the other variant names listed in the Wikipedia article for broccoli, and 西蘭花 (both traditional and simplified forms) was the most common result.
Apparently they categorise 芥蘭 and 西蘭花 as the same species… of different cultivar groups… although they do not at all taste in any way similar. For Chinese 芥蘭 and 西蘭花 are two completely different vegetables.

西蘭花 is broccoli, and I think it is okay to call 芥蘭 kailan.