I have a PCB with the default configuration:
When I export it into a STEP file, the PCB has 1.51mm instead of the board thickness from stackup of 1.6mm
Is this a bug? Or is there another parameter for the 3D thickness?
I think you didn’t measure (or export) the outer copper layer thickess + soldermask:
1.6 - 0.01*2 - 0.035*2 = 1.51
Note that there’s no true answer to what the thickness should be.
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This board contains a micro opto switch and the mechanics guy needs to check if it fits on in the CAD drawing, so precision is important…
Zooming in I realized that the components are floating over the PCB at 0.085mm distance
So this means that the 3D is done placing the PCB at 0 with thickness 1.51mm and components floating above it at 0.085 distance… which means that I have the opto-switch placed at the right Z position which is what I need 
The thickness of a PCB is not an important factor during PCB production. On top of that, you mechanical guy should know that precise dimensions and measurements do not exist, everything has tolerances, and when you make tolerances smaller, stuff gets more expensive.
You can of course attempt to find a PCB manufacturer that makes PCB’s in a “precise” thickness, and if you do, I wish you lot of luck with that, and am curious at the responses you will get.
The thickness of the solder between the component lead and the pad is an unknown
Just a note here. PCB heights are nowhere near as accurate as the positioning on the X-Y plane.
PCB manufacturers can have tolerances such as ± 0.14mm on a 1.6mm PCB.
Solder paste can also cause your component to further offset off the board.
after reading all replies I agree this is a false issue… it is not important to be 0.1mm precise because then we should also take into account the solder thickness, tolerances etc…
anyway I find interesting that, on the STEP 3D, the components float over the PCB so their top is theoretically on the good Z from the bottom of the PCB, only missing the solder thickness
I will measure the final soldered position of the components just for curiosity, but in practice we will use the PCB manufacturers tolerance for final mechanical tuning
There is no need to place an @ in front of their names. This forum software will ping them if you do that, and this does not have any benefit for them in this case.
And I was “first” with mentioning the tolerance thing. I would not say it’s a “false issue”. Tolerances are real and you have to think about how you incorporate them into your design to avoid production problems, especially for the “outliers”.