Why do custom millwork costs vary so much between quotes?

[David_Burns]

I’m currently involved in an interior fit-out project that includes a fair amount of custom millwork (reception desk, wall panels, built-ins), and I’ve noticed that pricing from different contractors can vary a lot more than I expected. Even when the drawings look fairly detailed, the quotes still come in quite far apart.
A cabinet supplier I spoke with mentioned that millwork pricing often depends heavily on the interpretation of details like finishes, hardware quality, joinery methods, and installation complexity. It made me realize how easy it is for scope gaps to exist in this trade compared to more standardized construction items.
While researching, I came across references to accurate millwork estimating services that focus specifically on detailing quantities and components before fabrication starts. It sounds helpful, especially for avoiding surprises later, but I’m curious how commonly this level of estimating is actually used in commercial interiors.
For those who’ve worked on projects with significant millwork, did you see big cost swings between bids? And what factors usually cause the differences? Would appreciate hearing practical experiences.