Argh, ok, I figured. I just randomly saw in a forum earlier this week that a few years ago, the templates used to be slightly different (they probably just didn't think that one through at the time).
Thanks, though!
They used to process all the visas in New York - and the emails from New York tell you what the decision is... whether the visa has been approved or refused.
However, in March 2013, they moved all the settlement visa processing to Sheffield... and the emails from Sheffield simply say 'a decision has been made', but they won't tell you what the decision is.
It's just a difference in how the two offices do things.
So, if you apply for a visitor, student or work visa, you will get the decision in the email, but if you apply for a settlement visa (fiance, spouse etc.), you won't.