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.
Looks like your psu has low voltage output that is why its only showing 3.7 amps instead of 7. Looks like the server psu is not compatible or the server psu you bought is only for gpu mining..
Maybe there is different voltage output or efficiency of the server psu is low.
I suggest you to use psu that recommends by canaan which is sorcerer psu v1 or v2