I find that not all trans, even among the same manu's and same trans itself are perfectly 10% linear trans on all taps. at 120v the 12v tap may very well read anywhere from 12.2 to 13 unloaded, 11.2-12.6 loaded, and at 108 volts it may not necessarily drop 10% on loaded or unloaded. voltage imput and amps used is not a perfect science as trans have different efficiencies at different loads, and at different imput voltages. and all manu's seem to set them a bit differently, and there seems to be an acceptable range within a given manu. if you grab 10, 300 watt trans from the same manu at the same imput voltage, you will still find various loaded and and unloaded tenths of a volt difference.
I would not lose sleep over it. accept it and try to volt your lamps properly regardless of imput voltage.