It appears that the issue is with the data, not with the SUM() calculation. Looking at IO 805420, three "Names" are shown under purchasing document 993873076 when there should only be one, and as you already noted the Val.in is repeated for each item. It is best to connect Tableau to the source data if possible. I don't know whether that viz is connecting to the result of a pivot table, or connecting to an Excel sheet with subtotals applied, but the data seen by Tableau are not aligning with the spreadsheet. I expect that the SUM() issues will be simple to resolve once this viz is connected to source data.
You're absolutely right and thank you for your observation. I'm connected directly to the excel sheet which is living in a network folder. The flow would be that the users will update the spreadsheet and those updates would carry over into my report and eventually into tableau server. I'll have to ask the users if they can separate this data out so that it ingests into Tableau properly.
Thank you for your help and clarification,