If you ask software development companies, plumbers, builders or lawyers to give an estimate of what some work might cost you'll probably get similar variations?
Building a wall of known size takes X bricks, Y windows, etc. Contractor 1 might have more markup or estimate different hours to complete the job vs contractor 2 but the parameters of the job are relatively fixed.
The dentist is seeing the same mouth (wall) and telling you there is vastly different numbers of cavities (bricks/windows). And not off by a small margin, but orders of magnitude.
Dentists and plumbers have what excuse?
The older the building, the higher the chances stuff will go seriously, sometimes catastrophically with no chance of recovery, wrong. Say for the plumber, according to the plans or the knowledge of the building owner, there should be no more lead pipes left in the building. Lo and behold, you open the wall, and find a lead pipe still under pressure, and next to it a tap to somewhere else that doesn't show up on any plan.
Endboss is if you find asbestos. That turns a jobsite worth a week of work instantly into a hazmat site plus immediate concerns for OSHA aspects.
Or for electricians, the plan was to rip out the entire electrical work in an apartment, and you suddenly find out that half the rooms of the neighbor next to you suddenly don't have lights any more because for some unknown reason half of his apartment actually runs via your client's apartment and no one ever noticed. (In this case it turns out that decades ago, this was one giant apartment that was split as part of a divorce proceeding and then sold off separately)
Tell me you missed the point without telling me you've missed the point.
Plumbers can give widely different estimates for the SAME job, with the same knowledge (initial or after finding out about all the asbestors, strange piping, and other issues).