Yeah, the whole "macro" thing is kind of a jackpot.
And then, it seems that people really enjoy seeing a fairly-detailed shot of a single blossom. Depending on the plant, that can run right into macro territory itself without much trouble.
For the record, I like more categories rather than fewer.
Also, by way of observation... no matter HOW good the description of a category is, people are going to screw up - either through ignorance (like I have a few times) or just plain human stupidity (I put a day lily into lilies this year - even though I knew better). So...to not get too wound up about it, I just expect that 80% or so of the people will get stuff right...and that's just going to be good enough.
The people who run the site really do have their hands full - and the contest seems to come off well-ish each year. And I have to suspect that, in the big picture, it's not the squeakiest wheel or hottest iron in the fire.
Good fun, though.