BECMI D&D has two overlapping bell curves happening. If you rolled a hypothetical 1d16+2 for ability scores in becmi, you'd still get a bell-curve for ability bonuses: you'd have a 1-in-16 chance of a +3, a 2-in-16 chance of a +2, 3-in-16 chance of a +1, etc.
AD&D (and I think OD&D) don't...