Yikes! It is surprising that yum operates so inconsistently. If you
same thing twice one right after the other you can get different results.
The thing that bothers me is the "false negative" saying that there is
nothing to update when in fact there is. But that kind of stuff is for the
"yum list" isn't it.
You should note that the mirror(s) you're connecting to are probably
actively changing state under you.
So please, instead of saying yum is operating so inconsistently you
might want to be aware of the inconsistent state of the repositories.