Why does a projected edge of a body not include the edge when extruding?

[mgianzero]

I'm simply trying to cut a corner off of a 3D extruded object.  When I turn on the object and create a new plane on one of its surfaces, then cut a piece off the corner by snapping the sketch line to each edge and extruding (i.e. cutting) the piece off the corner, F360 keeps a remnant of the outer edge.  Why is this?  

I can do the same thing by projecting the surface of this model into my sketch plane and draw a similar intersecting body and it does the same thing.  What am I missing?

Attached is my example model and photo of what I'm seeing.

[jeff_strater]

this is the problem.  These faces are not at 90 degrees - they are off by a bit:

[jeff_strater · Accepted solution]

this angle goes back to sketch1:

[mgianzero]

Thanks Jeff for the quick answer.  I really appreciate it.  I had redrawn my part using more of a profile view since there were lots of off angles that I felt were easier to draw this way.  Unfortunately, I did not apply enough constraints to my sketches to make certain things are lined-up properly.