Rotating trucks

Hi all. I did not found the same “problem” so I ask for it now. Perhaps it isn’t even a problem I don’t know.

So I am building the primo right now.
I put on the trucks, no rails for the core yet. Now when I move one truck all over the rail, the truck is spinning around the rail. Not much, but definitely. Like 3-5 degrees over the whole x axis (87 cm long, exposed rail)
it don’t when the rails for the core are in. Cauz it stops it from doing that.
Now, why does this happen?
Is this a problem?
if yes, how to fix this?

Thanks in advance for any help!

This is not a problem because as you said the gantry rails prevent it from rotating. It occurs because the bearings are not perfectly perpendicular to the rail. Usually the trucks should not be too tight on the rail and that allows them to slip without too much effort.