Some translations are not getting translated

Using Django Getting Started
1

Hello

I have this in my admin.py:

class SomeAdmin(admin.ModelAdmin):
    @admin.display(description=_('Cases %(year)d') % {'year': previous_year}, ordering='cases_previous_year')
    def previous_year(self, inv_presidency):
        ... some code ...

and this in app/locale/ar/LC_MESSAGES/django.po

#: case_tracking/gdoi/case_movements/admin.py:313
msgid "Cases %(year)d"
msgstr "قضايا %(year)d"

everything is getting translated, except the one mentioned above!!
why is that?

Thanks