Ogres traditionally had a Strength of 19 in 1E and 2E AD&D.
And yes, I believe it was to keep a "natural" ability score of 20 to remain special.
I was somewhat surprised to see the interpretations of some of the magic items, such as the DMG Manuals and Tomes allowed you to increase your maximum beyond 20.
Also personally I'd have the Belt of Giant Strength provide a limited boost (perhaps per day) rather than setting a characters Strength at a number beyond 20.