I've only got one item which is not up to date. It shows Reinstall and Update buttons. From this and the other combinations I see, my understanding is this:
Install = Download the currently not installed item and install it.
Reinstall = Install again the same version as the currently installed item.
Update = Download the newer version of a currently installed item and install it.
I'm not seeing a combination of Install and Update for an item at the same time, only Reinstall and Update. Are item(s) showing both Install and Update?