This is all about repositories. When you update using zypper it looks at all your enabled repositories. But some software packages appear in more than one repository. Let's say for example you have both the "Suse update" and the "Packman" repositories active. Then zypper might find an update for say gstreamer in both of them. So what's it to do? Well, because it "knows" which repository your currently installed gstreamer came from (say Suse update), it will only install an update from that Suse repository. But before the update it will tell you that it also found a gstreamer update in the Packman repository, but won't be using it. This is why you get a list of things which "will not be installed".
So it's not really a problem, it's just that there are multiple versions of the same packages sitting in different repositories. In the above example, if you disabled the Packman repository you'd find the "will not be installed" message would disappear.
To keep everything tidy, you should keep the number of active repositories to the absolute minimum you need.