Haha it is funny isn't it??
I think where you're a little off is that you leveled the face of the blade, but the "zero" is actually at the centerline and not the face. So when you measure the angle without the frame back you should get something other than zero. Given that starting point your going to get something more than what you're showing now, prob closer to the predicted 15 deg.
You can get the centerline level by measuring the angle of both faces until they are equal in magnitude.
Or you can trace it on paper and measure with a protractor