I think that there are more indirect behaviors that go into determining if a message is spam or not (given a filter misses it and it gets to your inbox). First and foremost, do I know the sender? That's a big variable that that quiz cannot reproduce or take into account - so of course people will have a tough time determining if a message is spam or ham. Second is the presence of attachments. If I know the sender, and the message isn't something like "check out this great video!", I'll be pretty sure it's ham.
That quiz is great for the basics and the practice of looking at headers, but I feel it misses the most fundamental aspect of knowing the sender and letting your brain do the work that filters miss.