How much do you have to lie before people think you're just a big fat liar who lies and lies all the time?
If you're the President of the USA, quite a lot.
I've never seen an administration like this one for the lying. It's like their policy is to lie, and then when they get caught about it, lie about that. And yet, notice the left-hand side of the graphic. The trusty old 39% of the populace (the same 39% that approves of Bush's performance) says, "Oh, yeah, he tells the truth most of the time."
If you like the guy, well, whatever. But to say he's basically honest is, well, dishonest. Or ignorant. Take your pick.