I work for an EPCM firm in YYC. When we do estimates we go out for budgetary pricing and the venders Are aware of this. This creates budget pricing that is inherently heavy (maybe by 10%). The quote get updated prior to construction. A good EPCM firm working in their niche will have multiple projects on the go and we know what line items are worth fairly accurately. Then at the bottom often the very last line, there is a "contingency" line item. Depending on the class of estimate (accuracy) this contingency can be 5% to 25%. Also as projects get larger your estimate gets more accurate by %. More room for high and low items to cancel out and 5% on a Billion is still $50million