There are 2 things to consider. I expect that since the net delivers 50A that you have a multi-phase DC-DC converter power supply. The DC drop analysis models each of these as ideal voltage sources instead of variable current sources, so it doesn't distribute the current evenly out of all the phases. That might be a big or small factor in what you see in the results.
The second thing to remember is that the traces or areafills in the power delivery net have resistance, too. The path from source to sink through each via is unique and has a unique resistance value. Therefore, the current through each via depends on the via itself along with the path resistance to the source and the path to the sink.