Since I'm not 1% gay, I can't speak for those guys, but I've heard some of them say that it represents to them that they are straight but open-minded. That's essentially the reason I put 1% straight in mine. Or it could mean that they are straight but secure enough to acknowledge that an occasional thought about a man being attractive has crossed their mind, even if they have no interest in acting on it. Like Talmidge said, true absolutes may be rarer than all the shades of middle.
In the end, the sexual orientation percentages are totally subjective - they mean whatever their user wants them to mean, and two people may mean different things with the exact same percentage.
Several people made good points about the importance of not judging people based on their percentages/orientation. And as Lex said, the same can be said about gender as well. Our profiles allow us the options of giving (or not) whatever information we feel like sharing about ourselves.
The OP was clearly just asking a question out of curiosity, not trying to cast any aspersions. But I agree that the lesson is that if we allow that 'identity information' to discount what we can learn from a person, it's our loss.