Unless you are familiar with building projects, you might know the difference in a budget number and an actual bid. A budget bid is not an actual bid, it is for budgeting purposes with the loan and finances. Most lenders require a budget bid which shows estimated costs for the project. The actual bid can change from a budget bid in many ways. The city approved project drawings could have added several things that the budget bid did not include. Then the client or architect could add certain changes to materials in the actual drawings from what was budgeted off of.
Most general contractors budget bids are done with minimal drawings and usually no actual product details. Which can greatly affect the cost when you get to the build phase. It is always in the details and that is what tends to separate a budget bid from an actual bid. A bid number is typically taken off an actual set of architecturally stamped drawings and permit approved.