Boolean Union_changes the surface

Hi there,
I have done a Boolean Union between 2 surfaces, and after completing it, one of my surfaces (the joint) has changed a little bit.

Any ideas about what I am doing wrong?

Many thanks.

First picture: previous to BooleanUnion

Second Picture after doing BooleanUnion

And these are the Surface curves, prior to the BooleanUnion:

That’s just a difference in the appearance of the render mesh, it hasn’t actually “changed” anything. Usually Unioning objects–well do you mean Join? This doesn’t look like a boolean? Same thing either way anyway–will make the meshes denser…but apparently not in this case right there in that spot.