I'm going to guess its the difference in power supply efficiencies. 7 vs. 6.3 is only 11%, well within the range of that.
There are 2 hot lines for a 240v circuit. So a PSU rated at 7.8 amps @240v should be showing 3.7 or so on each line, not 7. The 6.3 seems correct for a PSU rated at 15 amps.