Boolean Union_changes the surface

Rhino Rhino for Mac
1

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

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Captura de pantalla 2019-01-22 a las 1.28.34.png922×901 35.8 KB

Second Picture after doing BooleanUnion

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Captura de pantalla 2019-01-22 a las 1.29.07.png943×890 37.4 KB

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

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Captura de pantalla 2019-01-22 a las 1.36.32.png989×788 79.1 KB

2

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.