As someone already mentioned, there are many different varieties of sunflower - if they are native to High Isle, then you'd expect only one variety. If they're not, yearly cross-pollination would mean that you'd end up with them eventually becoming one variety. I'm ok with them being the colour they are. What I don't like is what someone already mentioned - that they don't all face the same way. Sunflowers were named sunflowers because they follow the sun throughout the day. I don't mind if they aren't tracking the sun's movement, as long as they all point roughly south. Big goof imo.