The first screenshot (window title "Ubuntu Software Center") shows the old Ubuntu Software Center (package software-center
). It comes with Ubuntu up to version 15.10.
The second screenshot (window title "Software") shows the new GNOME Software Center (package gnome-software
). It comes with Ubuntu since version 16.04.
The reason why you have both software center applications installed is probably because you upgraded from 15.10 or an older release to 16.04 (or later). That way the new GNOME Software Center gets installed, but the release upgrader does not remove the old Ubuntu Software Center, as they are completely unrelated packages. It simply does not know that one should replace the other - not sure whether that is a bug or a feature though...
If you want, you should be able to uninstall the old software center using this terminal command below:
sudo apt remove software-center
But please check the list of packages which are going to be removed with it! I can't tell whether any of your installed applications might depend on that package. If there are packages in the list of remove candidates which you definitely don't want to remove or about which you are unsure, it's better to leave a comment and ask for advice than to blindly remove packages.