If you do too small of an area the 80% overlap isn't really doing its job. The point of doing 80% overlap is that everything gets 25 looks at it for the processing. Your map only had 3 rows of images so the most anything was ever going to get was 15 looks.
You need to have 25 looks at everything if you want to have accurate 3D models. An accurate 3D model is required for doing solid volume measurements.
You also want to make sure that all of the images are collected while travelling at the same speed. Map Pilot does this for you in Active Connect mode. I am not sure what app you are using but you might want to look at that.
Also, make sure you are using lots of points to draw your measurement polygon (~20, not 4).