Solution : https://service.sap.com/sap/support/notes/827019 (SAP Service marketplace login required)
Key words : 
actual undo space requirement, high water mark, relevant tablespace alerts, relevant tablespace alert, terms br tools, transaction db02 differs, transaction db02 displays, actual space, transaction rz20, space allocated
Related Notes : 
       
| 1036513 | Corrections in RZ20 for Oracle monitoring (8) | 
| 825192 | Corrections in RZ20 for Oracle monitoring (5) | 
| 680046 | Corrections in BR*Tools Version 6.40 | 
| 659946 | FAQ: Temporary tablespaces | 
| 514205 | Corrections in BR*Tools version 6.20 |