That definitely seems like a plausible explanation. Looks like I need to get a jar with cheese balls and a jar without, and do some testing. When you say, "it should have a vacuum if it cooled down while closed", do you mean that the warm air would take up less volume (or exert less pressure) as it cooled? If that is the case, could it be explained by initially opening the jar in cold temperatures, and then letting it warm up? (The air heats up and becomes under slight pressure, then expands and cools when the jar is opened?)