Gallup’s sample seems to hold The One in higher esteem than other polls. Dunno why. So that makes this good news — that in the only remaining poll with remotely good news for The One, he is now underwater.
The reason is that Gallup polls a sample of "all adults." That includes illegals, lifelong moochers, and other sorts who either arent registered to vote or rarely show up. Such a sample will typically be more lefty than samples of registered or, better yet, likely voters.
From the standpoint of what will happen in November, what all adults think is minimally meaningful. It's what likely voters think that matters, and likely voter polls show him with even lower approval.
Why does Gallup do this? It either wants to skew the sample to the left or is opting for the cheapest polling method. Or both.