Maybe there will be a woman on the Democratic presidential ticket. That doesn't mean it has to be Hillary:
The critical thing is that women are both planning ahead and planning for contingencies. Hillary may or may not run; I think she will, but not everyone agrees with me.
Hillary may or may not win the nomination; I think she will, but there are any number of scenarios in which she could lose one or two early contests, an anti-Hillary movement could develop, and someone else could take off.