Another view is that this is the maximum difference over time.
No that's incorrect. I get that you don't like Obama but do you think you could stop polluting the thread with careless factual inaccuracies? I get stuff wrong too sometimes, but I feel like you're just being sloppy at this point.
The maximum difference over time was ~40%
in Bush's favor about 1 year into their first terms, when Bush had ~90% post-9/11 and Obama at the same point in his presidency had ~50% (or thereabouts).
The fact that Bush converted his 40 point lead into a 20 point deficit, or a swing of 60 points, is about as significant as these things can be. And it's not like Bush had an unlucky downward spike right at the end, his final approval of ~30 was the culmination of a steady, downward trend for 7 years. The more years of Bush and Obama the country experienced, the more popular Obama became relative to Bush - big time. Deny that approval ratings prove Obama was a better President if you want (I certainly agree with you there), but the difference is not "slight".
Averaging, the difference is less.
True, and interesting, but not as meaningful for the reasons stated above.