If these are sealed glass bottles, evaporation is highly unlikely. What is in the bottles, some type of oil or other foodstuff? You don't give any indication of the time between shots. I assume that these are tripod shots and if so, are you sure your tripod was unchanged between image capture? Depending on the liquid, the type of temperature change that would be observed in a 'normal' room is unlikely to do anything major. Could just be an optical issue.