Driven Subie's for years, and this has happened to me before as well. Not really sure why. I've just learned that if I get a low reading unexpectedly, to wait until another drive cycle is complete (as long as the level is safe) and check again after shutdown/draindown on a cold engine. 99% of the time the oil level seems to correct. I'm not sure where the hidden oil goes, and why it only happens randomly, but it is "normal."