A small update. I've checked if this behavior is the same in Ubuntu (12.04), and it is. The system-set default gamma (used xcalib and gnome color manager - same results) matches that in Windows. I've also tried dispcal -R, and it gives 10 bit, but that's for uncalibrated curves, so I'm not sure if it's even relevant. So, I guess the question still stands, if this discrepancy has any effect on the calibration and profiling processes via Argyll.