Could a PC consume more power from the plug if it oparates in a colder enviroment?
Many modern CPUs and GPUs automatically scale down their clock speed and/or number of operational cores if they are too hot in order to prevent dangerous overheating. This could well be the cause of your discrepancy; if it is (which you should be able to test by monitoring your number of hashes per second in different conditions) you may need to install additional cooling.
While it is perfectly possible for either your consumption to change a little without this happening or for your meter's calibration to drift as temperature changes, I would expect both to show the opposite effect,i.e. reading higher power consumption in warmer conditions, so I don't believe this is likely to be the case.