Sounds like the meter is not perfect. Or it's battery is low. Still good enough for your test though. It could be intermitent leads or oxidized plugs but think not as the given values are pretty close.
Anyways consider the oscillation as a meter problem for practical purposes. You are looking for low values on each plug. Usually a burnt out plug will be wide open. You still might notice a plug with increased resistance . The oscillation range should vary if there but at slightly higher values when it settles down.
As you say not rocket science but confusing to some exrent depending on amount of experience using a meter. Usually the worst problem reading things is getting good clean connections with no oxidation on your points of contact.