What is the biggest reason commercial estimates go over budget after the calculator stage?

The most common driver we see is scope that was never firmly defined at the estimate stage, particularly around MEP specification, fire compliance requirements and façade ambition, all of which get resolved later than structural design and often cost more once they are finally locked. A close second is approval delays that extend financing and holding costs beyond what was planned. Locking scope and approvals as early as realistically possible protects your budget far more than negotiating a slightly better headline rate.