How does this happen?

Both done with sketchup and one is imported into the other.

When moved together they don’t line up.
How does this happen and how do i fix it?

Hard to tell without seeing the .skp file.

A common cause of this sort of thing has to do with having Length Snapping enabled.

Looks like there might be some bad geometry in there.

Why would you model the parts in separate files? If you modeled them together you’d have a better shot at things lining up and it would be less work.

As Dave says you have some inaccuracies in your model that are being hidden by your Display Settings for Units.
Adjusting the unit settings only affects what is displayed and has no influence on accuracy, SU is as accurate as you are, and length snapping can actually be detrimental to accuracy.

You can see here, the geometry is obviously wrong but the Dimension shows it should be correct, but it is simply down to the dimensions rounding to the option you have chosen.


By the way, all those dotted diagonals in your images are showing that your faces aren’t flat.