I've been preparing budgets for a number of new clients lately, and I'm often finding that assessments have been charged in the past do not correspond accurately to the ownership percentages recorded in the association's Declaration.
When I purchased my first condo and prepared my first budget as Treasurer for the building, I noticed that some owners' assessments increased while others decreased. The only reason for this to happen is if the original assessments have not been calculated based on ownership percentage, as they should be. If the total budget increases 10%, each owner's assessments should increase 10%. It's simply math.
Every time a budget is prepared, the ownership percentages should be checked against the Declaration to ensure that owners are being charged correctly, particularly if you are a new Treasurer. This will ensure that every owner pays in their proper share.