As Jeff said- the announcement of the accounts report is made at the Sunday morning announcements. Most of the donations come in Sunday at lunch or Sunday at the end of the day. This is just typical human behavior- procrastination. The accounts report provides a chance to remind everyone that they should donate and the deficit scares those who already donated into donating more. All assemblys have a deficit on Sunday morning and no one in the administration is much concerned about that because they know that Sunday lunch and after the session is the big take.
Regarding the expenses, assembly halls have a "required donation" per attendee ($4-$10) based on their operating expenses. The total "cost" is calculated from the attendance and that is put against the money collected as of Saturday. Therefore- automatic deficit. Additionally, as Jeff said, the elders usually pass resolutions at the Saturday PM meeting to donate lump sums to the Worldwide work, reimburse expenses, etc. These resolutions are often passed with a deficit showing on the accounts sheet. The resolutions are based on what has been collected in that circuit in the past. So, if there wasn't a deficit before- after these resolutions there is one. Or a deeper deficit.
Thats how it works at the circuit level. At the district level- I think a lot more of the $$ goes directly to the Worldwide work since often they get the arenas and facilities for free. They're free because part of the pitch that the Convention Office makes to the town, city, chamber of commerce is that we have 5000 JWs coming for a once a year program. They will spend $x in your local economy on food, lodging and gas. That will turn over x times in your economy, generating x dollars in tax revenues and x jobs. So- give us the arena for free or we'll go somewhere else.