Under BX/BECMI, it's not a d20+ mod, it's 1d20≤ Att for many checks. It's only attacks and saves that are 1d20+mod.
AD&D, NWPs are rolled 1d20 ≤ attribute+extra-slots; same for BECMI/Cyclo General skills.
Because the Rules strongly imply I can. In BX, it's in the example of play that actions...