That's his true self. When he usually gives praise or credit, which is not often, he tends to adorn it with a subtle dig. With Federer there is always a but...
I've often read that, I partly understand it (he's been known as someone who likes teasing for instance, it's part of his character) but this idea has reached such proportions that it has got out of control in many people's minds : these people see him so much as a devil that anyway they will always read his words like that
I disagree with the idea that he always speaks like that :
1. he's not clever enough
2. he gives many interviews and has other things to think than always playing such games
Fed is also the guy who said that he regrets some things he said but he "can't see the point if one is not allowed to speak one's mind in interviews". He's honest and less cautious about his words than other guys.
Everyone has his own speaking style and some don't like others' speaking styles, I think it's a strong reason of that allergy many ones have about Fed.
For instance, I know many ones who are allergic to my speaking style
It's harder to know Djokovic's and Murray's real self becausde they're much more cautious when they speak since past problems they had.