Some of the categories are tougher than others on purpose. It's like achievements in games; you get those first few easy ones for finishing the tutorial or making your first kill, finding your first collectible, etc. and it gives you a little feeling of progression or accomplishment, but then there are the ones that you know are going to be extremely tough to get, or will take some time. It's good to have that mix.
The contest on the other site that I borrowed (stole?) this idea from was like that, too. They had 12 categories as well, and there were a couple I just could not get. Scoured my backlog with no luck.
Out of curiosity, I looked up my achievements so far for the month of May to see how many categories I'd unlocked without trying to. If I were someone just stumbling across this contest now, I could fill 7 of the categories for sure, with one other iffy one that I'd have to plead my case for. That's why I hope even if people are just seeing this now, they'll still give it a shot, even if they only get a few of the categories.