When the budget is set up ~ say at 100 mils to produce $400,000 (a specific amount) and it produces $432,000 in revenue; it over-collects $32,000.
That $32,000 by law 12-43-285 has to be credited to the tax payers or given as a credit on the next tax bill.
Another example is if the revenue is said to be 98% collectable (accounting for non payment of taxes) and it actually is over 100% (because all properties are charged fees if late, or sold at auction of unpaid), that means there is another $8000 that should be refunded to the tax payers.
(2%+ X $400,000= $8000+)
The same is true at reassessment time. The School Board or the County Council can not benefit from reassessment according to law. Put this in terms of $30 million and that is a quick $3,000,000 in over-collected funds.
The money is not put up for a rainy day by the school board, it is spent on things not necessarily needed the same year, or in board terminology "found" for whatever pet project need or special interest appeasement they choose.