you are not obliged tu put the ellipse on the surface for make a boolean diff ( keep surface) or a Trim (don't keep surface) !

But as your object is actually not a solid , the surface is not created with boolean Diff!

In your case I suspect that you have taken one axe of the ellipse in the 3D space so no // at the surface, so inclined! ;)
Move it on the surface and you will see that is 0.972769° difference!

All that because your object is floating on the 3D space and not alligned to the gride ;)
You can of course don't use the gride but it's very more difficult! :)

You can use the Menu :Vue/"Plan Aux" (CPlane) for temporary use ;)

Not many interest to have perdicular view in 3D view!
With the CPlan actived from your face wanted you have that in the Topview(Cplane) ;)