Question about ducks in my pool?

Okay. I have an in ground pool. Every years ducks come to swim and we feed them. Well last year there was a female mallard, male mallard, and a male black duck. This trio would always come together, and the black duck would chase off the mallard from the female. Well this year the three come together and the two males are not fighting. In fact she seems to be spending equal amounts of time with both males. Is this unusual? The male green headed mallard has gotten bigger than last year, so the black one and the green headed one are about the same size now. Is that why? How can she have to mates?