Can you repeat your tests because they don't seem to make much sense ? I assume you're using exactly the same motor and prop all the way through and each battery is freshly charged ? In which case what you have is
Test 1 20A @ 11V (on the 2200mAh 20C). O.K. that's a reference point
Test 2 15A @ 10V looks o.k but that's a much weaker battery (max 22.5A rather than max 44A in Test 1).
Now we get to
Test 3 18A @ 11V. With the same load (motor/prop) it shouldn't be possible to get less current at exactly the same voltage as Test 1. If it's running at 11V the current should be 20A again. If the current is really lower the voltage should be lower too.
Test 4 17A @ 11.5V. Even stranger you're now showing a HIGHER voltage but a LOWER current. That's not possible either if the load is still the same as in the other tests.
Something odd going on but perhaps it's not just the battery performance