kVA is usually not a simple summation of 2 kVAs.

The kVA on 2 summated meters are usually lower than the simple summation of the 2 kVAs separately calculated (and usually at a different time as well).

Consider the chart:

Here you see a kVA of 68 (A at 7:00) + 123 (B at 10:00)=191

but the summation comes to 180 (Summation at 13:00)