Weird measurement bug, or is it me?

Hi All:
I’m getting better at this, I swear I am.
Now I’ve brought my house plan into Layout, to assemble a presentation for the contractor.
Three times I’ve used the “Linear Measurement” tool to do the seemingly simple task of measuring the size of the rooms.
Three times I get different answers.
I click the same points on the drawing each time.
I have changed the scale each time. The way I understand how this tool works is that no matter what the scale of the drawing, i.e. the size of the final rendering on a piece of paper, it should be measuring the actual distance between points on the project at real size. Do I have this right?
My evidence follows. What the heck is going on?
Not only are the measurements wrong, they don’t even add up correctly.
(The actual intended sizes are in Sample 3, .375"=1’ scale.)

It measures based upon the scale you select. Auto recognizes the scale set to the viewport.