Put two competing bids for the identical scope of work side by side, and the gap between them can be startling. Not a few percentage points — sometimes a difference big enough to make you wonder if one of them read the plans wrong.
They didn't. In almost every case, the gap comes down to one of three things.
What's actually included versus assumed
One bid explicitly prices out a piece of scope — say, disposal, or a specific finish detail — and the other simply doesn't mention it. That's not always dishonesty; sometimes it's an oversight, sometimes it's a deliberate low-ball that assumes it'll get added later as a change order. Either way, the client comparing two "final" numbers has no way of knowing the bids aren't actually pricing the same job.
Waste factor
As covered in the first piece here, waste factor is one of the most inconsistently applied numbers in estimating. A contractor pricing tight — assuming close to zero waste — will always underbid one who's pricing for the reality of a job site. Tight numbers look better on paper and worse three weeks into the project.
Whether the number came from a real takeoff or a gut check
This is the biggest one. A bid built from an actual measured takeoff — real quantities pulled from the plans — and a bid built from "this feels about right based on the last job like this" can land in completely different places, even when the estimator doing the gut check is experienced. Experience helps you gut-check accurately. It doesn't replace an actual quantity.
The dangerous part
The dangerous part of all this isn't the low bid itself. It's not knowing which of these three reasons explains it — because two of them are perfectly reasonable, and one of them is a problem waiting to surface.
If you're the one submitting the bid, the fix is making sure your own number is always the third kind: a real quantity, not a guess dressed up as a number. If you're the one comparing bids, the fix is asking pointed questions before assuming the lower number is simply the better deal.