I know for me that if I don’t keep a thermometer under my tongue after the beep goes off for about a minute, my temp will read about of few tenths to half a degree lower. Also, my temp will vary by where exactly I hold the thermometer under my tongue – I wouldn’t have thought it would make a difference but it does and a half degree is significant, at least to me.
If it isn’t that at all, I am unsure of what is going on but wouldn’t worry too much if you don’t have the other symptoms of low metabolism.