Yes this is normal and more noticeable in AIO's then sumps. Your chambers are going to fluctuate. Unless its making major salinity swings (it shouldn't). Don't stress. If you want less fluctuations in water level, you need the fleece roller to roll more frequently as to not let it clog up. But this comes at the cost of wasting material.
How it works: The fleece obstructs water flow as it clogs making the return chamber lower. So now your ATO will trigger to add more water. Eventually your fleece roller will rotate to clean fleece. This increases the flow and allows more water into the chamber causing the rise again. It will eventually equalize. You just need to find the sweet spot.