I have LibreOffice 4.2.5.2 (in Italian, with the related help pack).
My OS is Windows 8.1 Update 1, in Italian.
(If I wasn’t subscribed to the newsletter, I didn’t have known about the 4.3 update)
As far as I know the available update notification works only for the same branch. This means if you run x.y.z you only get indication for an increase of z but not for an increase in x and y.
What I can imagine is that you get an notification for an change in y if the version you have installed reaches end of life.
As LibO is developing so fast, I look once in a while in the release plan and thus I know typically in advance what is coming up and when. Release plan: https://wiki.documentfoundation.org/ReleasePlan#cite_note-1
I didn’t know that.