As far as designer intent goes, I don't have a "smoking gun" statement by Mike Mearls or anyone else, and frankly I think it's rather unrealistic to ask for one. However, I can give you some clues that at least pique further interest.
If you've ever played AD&D, you know that it was far different from both 3rd edition and 4th edition. I think it's safe to say that it's common knowledge that 5e intended to bring back elements of AD&D that made it great, without bringing in the things that made them iterate to a new edition in the first place.
In AD&D, Gauntlets of Ogre Power behaved differently -- they gave you, directly and with no fuss, the actual Strength value of an Ogre, which was 18/00 at the time. In 5e, an Ogre's Strength value is 19. The pattern here is obvious. In fact, it goes further than that. As Jason K. mentioned, the Belts of Giant Strength also have odd-numbered values, just like the monsters. Hmm.
As for "why," well, I can only speculate here, but to me this is a clear attempt to bring some verisimilitude back into the game. What is that? Well, in a word, it's "believable-ness," or the appearance of being true or real. Things that allow you to maintain your suspension of disbelief quite easily. I haven't really played 4e, but in 3 and 3.5e Gauntlets of Ogre Power were a simple +2 Strength enhancement item. This is three things: 1. boring, 2. not balanced well, and 3. nonsensical. Why would a godly Barbarian with 30 Strength benefit from "Gauntlets of Ogre Power?"
Hope this answers your "unanswerable" question.