Having had recent experience in a new strata building, albeit a commercial property, I can say the initial budget , usually prepared by the anointed strata manager, is woefully under budget.
What has surprised me for new buildings is the amount of money that needs to be spent on certifying on an ongoing basis all sorts of building equipment ( afss, co sensors, height safety equipment, plumbing etc etc)
It’s taken about 4 years to identify and budget for all this.
Whilst the strata manager should have budgeted for these things, it’s also fair to say that every building is different, AND like you they are dealing with a non existent building till it’s finished.