Just looked at original more closely on my laptop previously I was measuring on my phone with my fingers.
You are right it isn't quite to scale: horizontal 10 yards is a bit longer (edit: shorter) than vertical 10 yards (edit: actually they are really close) Plus, the horizontal axis goes to +/- 30 yards, so I'm not sure what they did there it should technically go to +/-26.75 maybe they just rounded up to the nearest 10. At any rate, even with any scaling chicanery it would be an ellipse if it just depended in distance.
Edit: Plus they are not that different: horizontal 10 yards is like a couple percent different so it should basically be a circle. At any rate I think it is pretty safe to just use distances on the plot and not worry about this crap.