Why WBS PRPS-PSPNR right-aligned in ALV Grid by default? (field of type NUMC but conv exit to text)

Why is PRPS-PSPNR right-aligned by default in ALV Grid? (simple demo program below) I understand that PRPS-PSPNR is of type NUMC (number as text, which cannot be summed for instance, like a ZIP code), but it has a conversion exit which transforms it to text. Code: REPORT zdemo.