The WB is very nice, but, when you can log along with rpm's it becomes invaluable. Also be careful, the wb tends to spike during initial acceleration and during shifts, therefore, making it hard define the true weighted a/f.
As for your other question, "what can change the a/f", sure the temp, humidity and barometric pressure can alter the a/f, but PCM itself has a defined matrix that will also alter the a/f, therefore at times you are trying to adjust for the environment and the car is fighting you because it is trying to offset what it sees.
