The obvious answer to this question is yes.
Whether it's because it becomes harder to jb, Apple starts spitting out new versions faster or a combination, yes, a new version of the OS could come out before the last one has been jailbroken.
I've wondered how many people Apple devotes to doing this because what business benefit is in it for them. If they can close a hole as part of doing other development, they probably do, but why would they expend development dollars for this? Will they claim they are closing holes for PR reasons? Yes. Will they actually devote resources? Who knows.