This happens only when ST2 itself checks and finds a new version, not if I choose Check for Updates in the menu.
I don’t know if you use different update frameworks on the different platforms, but are you sure there isn’t a template for OS X somewhere that says "$CURRENT_VERSION has been downloaded and is ready to use!" instead of "$NEW_VERSION has been downloaded and is ready to use!"?
(I have a feeling I have uploaded a similar image before, but I couldn’t find it - apologies if I am repeating myself.)