1st.
They ask for SEA LEVEL.
If you breath 100% ox. with no pressure at 40.000ft, it will correspond to 10.000, not sea level.
2.
air driver AH is subject to acceleration as well, so it may/will show a slight climb when you are accelerating in level flight.
3.
An experienced pilot is less aware of what he is doing, it all automated.
So for an experienced pilot it will be a lot easier to overlook something VS "new" pilot, who is still insecure and very aware of his actions.
You can as well call it "routine errors".