Test Tone Discrepancy

Have noticed while calibrating my system that a -18db tone generated by my Cantar, a high end digital film location recorder, is played at -21.1db in Nuendo. Have pondered and checked this for a while. No effects on channel, faders all at 0bd. Nuendo’s test tone generator does create a -18db tone that reflects accurately but this does not change the fact that an imported -18db audio file from the Cantar shows at -21.1db on Nuendo. Any ideas besides it is not a -18db tone? The Cantar is a french machine, are db scales standard throughout the world?

Where does it show the -21dB? On the audiotrack itself? Or perhaps on a stereo output bus?
Discrepancys of 3dB often seem to be caused by pan law settings… And due to this they are mostly correct. Are you sure that you can rule out this?