I haven't read all 44 pages, so something similar might have been posted already.
Anyway, I noticed several things while playing around with level and compass on my 5S (week 39).
First of all, level's off by 6° flat, 3° long side and 0° short side. I then played around with the calibration on the compass by killing the app repeatedly and recalibrating on the next start. What I noticed was that at the beginning the red ball would go round just fine and as expected.
The more the app calibrated, though, the harder it was to get the ball to the top until it finally sits within about 90° at the bottom of the circle and refuses to move further upward, no matter what I do to the phone.
I also noticed a bug where I could get the level app to completely freak out simply by moving the phone around a bit from flat to side to flat. There is point where it simply refuses to go back to the "two balls" when placed flat on a surface and holding it upright would cause the screen to get half black, half red and with a white corner.
What this suggests is that the sensors do read proper data at some point (the ball behaves as expected at the beginning) and it might actually be a software error misinterpreting the data in the app, which is noticeably buggy.