My assumption would be that the gap between Male and Female is much thinner than to the Hardcore option. Thus it's a new option group. And an option group with only one choice is invalid thus is has to be a checkbox.
I just assumed the option for normal difficulty was to the left and that's why the image is weirdly cropped like that.
There is a gap between the male and female grouping and the hardcore. This is what leads me to believe that the hardcore option is in a different grouping, probably horizontal grouping which is why I think the normal difficulty is to the left.
After the first UI update pass, someone on the forums made this into a meme after they noticed that Crate missed the title screen in the update. The dev team thought it was funny and just left it like that.