The objective of using volume based SST instead of area is to reduce some of the noise and to have a better comparison to Ocean Heat Content and paleo Bottom/middle Water Temperatures. So the volume based total of the masked major basin areas will not be a perfect match for "normal" SST for a variety of reasons.
There is not a huge difference between the area and volume calculated basin temperatures.
With the volume weighting for just the five major basins I should be using only 87.1% of the total ocean volume. Since I am using 65S-65N instead of 60S-60N I am actually using more, which is part of the larger error in the comparison first comparison. I am not concerned with that error because it should show up as more "anthropogenic" warming/cooling or uncertainty in regions that don't have stellar data to begin with. The "rough" estimate of OHC is not going to be all that accurate anyway and is mainly going to be compared to paleo which has a much larger uncertainty range.
When I compare with Hadsst3 or ERSSTv3b, the error is larger and has a slope.
So just for early warming, the "temperature" anomaly I am using should not be directly compared to "Global" anomalies without consideration of the differences.