It's all a bunch of re-baked accounting and mind wash.
They always announce a surplus. Why?
So that the dubs are led to believe that whatever the costs announced last time, the monies were received in full... and extra. This is to motivate giving. If JW's were allowed to believe that the convention was shorted, then God's blessing would be lacking. So, it has to look as though Jah blessed it and it overflowed. But not too too much... don't want to let the dubbers think they can slack now and rest on their laurel$.
When they announce the figure is say $10,000 I am sure they don't get it. Probably get 60% or so. Still a good haul. But they cannot then announce a deficit of $4000 next time. How would that look?
The first thing the dubs would ask is 'how did we make it through without paying our bill?' The next thing they would ask is 'If we made it through without paying our bill last time, can we do it again?' And then they would ask 'if others aren't paying up... then why should I...?'
Then there's probably a host of other things that would go through dubber's minds. All of which would prove to be a negative for Watchtower's receivables.
And speaking of receivables... THAT, my friends, if why they NEVER EVER announce that one delicate number.
Amount received at our last assembly day.