Unless you were expecting to find oil in the plug wells, you wouldnt really be looking for it. So you remove the plugs and any oil in there runs down into the cylinders without you knowing it.
When you come to fire up you are burning off the oil that has run down. Until thats done the mixture is well out and it runs like a pig. Its pretty much what you get if you have introduced a little oil into cylinders when doing a compression test. The tell tale for you is if your socket has oil on it and if the old plugs' shoulders and threads have oil on them.