inconsistent results when rotating 2 objects

[sjskhalsa]

I followed the tip for creating an inclined roof with windows, and used the rotate about horizontal axis after importing the wall to set the angle. When I add a beam, drawn as a box, and rotate it about the same axis by the same angle it does not match the angle of the roof! I have to add 10 degrees to the rotation to get it to match.
Can someone clue me in as to what is going on and which object is oriented correctly?

[Puybaret]

Did you draw the exported wall horizontally to ensure that once imported it's not rotated?