Perhaps oil spilled over the side as you poured the oil in (?). As you likely know, the oil cap is elevated off the valve cover so when I changed the oil in my 98' C280 I removed the engine cover to better see that area and to quickly clean up any oil that did spill. If you did spill some oil there, it may take a while for it to flow down the side of the engine.

When you removed the oil, did you measure the amount removed to ensure you got most of it? If not, do you think you may not have removed all of the oil and then subsequently over filled the engine?
MarkC