It's very uncommon for a construction project to finish exactly as forecasted without any changes having been made.
Changes to the project scope and/or timeline are a constant irritation and occur for a multitude of reasons including weather, lack of materials, labor shortages, and even just the owner changing their mind.
As the general contractor, it's up to you to review the change requests and decide whether each one is necessary, and if so, who will bear the responsibility for the changes to the budget and timeline.
This template will help you capture the right information to permit solid decision-making.