Solution : https://service.sap.com/sap/support/notes/667288 (SAP Service marketplace login required)
Key words : 
terms query currency unit reason, 0 usd  + 0 euro = 0 usd rules, 0 euro + 5 usd  = 5 usd, 3 euro + 0 usd  = 3 euro, 0 euro + 0 usd  = 0 euro, 3 euro + 5 euro = 8 euro, currency/unit', 3 euro + 5 usd  = 8, query attributes, key figures
Related Notes : 
       
| 984133 | The operators NDIV0 and NOERR delete unit and currency | 
| 589260 | Units/currencies lost for values = 0 | 
| 586572 | Unwarranted display of '*' or 'X' as unit/currency |