Mismatch between weight calculated through dynamo and Schedule weight

I’m trying to automatically calculate weight from Volume of walls(2 walls within one assembly)
the weight in the schedule is different from the one I get from dynamo.

in the schedule I have a calculated value ( Volume * 2600 kg/m3)

the weight for 0 and 1 index from schedule corresponding to the final output in dynamo.

I think the problem is the Math.Round node in Dynamo, from a conceptual point of view: if you round your numbers in the middle of the operations, the final result will be different. Remove that node, or use it only at the end.

The value you see in Revit, i.e. 0.61 m³, is the round displayed value, not the value used for calculation. You can change settings to see more digits in your schedule.