More specifically for this year, the budget approval happened within the context of a debate over whether we could afford a DebCamp or not. To protect the main conference events, "worst case" budgets were assigned to them. That doesn't make it automatically responsible to aim to spend those worst case amounts.
(Since the costs were independent estimates allowing for overspend on individual items, the trading money between line items isn't automatically justifiable. For example, the dinner item was intended to include the cost of buses etc. for a separate event on a different day from the day trip, so combining the two should definitely lead to significantly reduced overall costs for the two.)
In future I suspect we should more clearly separate some kind of "planned cost" and "contingency" for such items. In most previous years all involved made a continual effort to keep costs down, so it didn't seem necessary to add extra stages of bureaucracy over this.