A couple things to think about.
One, I find the refractometor to not be an extremely precise instrument. If for no other reason than the graduated lines and the refracted blue line bring some in accuracies.
Second, the ambient temperature around you makes a difference with the device. While the refractomotor is “ATC” or auto temperature compensating, it does so because you place a very small amount of wort onto the device, and that essentially cools the wort instantly. If you are like me and take reading in the summer for the OG outside in 95F heat, and then take the final reading in my basement at 68F, the device will have some error due to temperature. (I try to take all my reading in my basement, which is consistent in temperature year round due to this)
Third, the wort tends to stratify sugar layers, so the wort at the top may read 20 brix, but half way down the reading may be 25 brix. It is hard to be sure your wort is the same all the way through. If you take a reading just after or during the boil, that should reduce the issues a lot. (Probably already your practice)
Fourth, most refractometors are only accurate to +/- 0.2 brix. So they can be almost a quarter brix off when functioning correctly.
Lastly, I think your sheet may be off a bit. When I use mine at 20brix I get 1.078 starting gravity, and with 10 brix final, I get 1.015 gravity. That combined with some possible calibration issues you listed, and some rounding or accuracy in reading the refractometor, I think that may possibly bring attenuation back near 75%.. Perhapse mine is off, but at the very least, I findi it interesting that they are different.(Mine is from more beer too)
Regardless, the refractometor, with the small scale, is not a very precision instrument. If you want to get accurate readings, the hydrometor is the way to go. However, if you are looking for trends or “just close” readings, the refractometor is perfect. I rarely use my hydromtoer any more, but that is also because I figure the beer is going to ferment how it wants to, and the gravity reading are more or less just data so I can reflect on what is different about this batch than last batch.
Good luck and I hope the brew turns out great!