On my last brew day I checked and re-calibrated my refractometer before starting the mash. (I adjust it before every brew) I then thought my OG was low at 1.046 instead of the predicted 1.053 so I checked and it was off again. I re-calibrated and took another sample and got 1.052. Quite a difference.
Often it will vary by the amount you have mentioned between readings just based on temperature. You can often take readings from the same pipette 5 minutes apart and get the kind of variance mentioned above.
but distilled water at room temp should be done periodically to calibrate.