After installation from an iso-image first thing to do is go to softwarecenter an look for updates.
Maybe you are wondering why there are a lot of updates available, when you did a new installation from an iso-image.
The answer is, that nethserver-iso is based on upstream CentOS 7.2.1511-ios and that this image was released about one year ago. But CentOS 7.3 was just released and in near future nethserver-iso will be based on that newer iso, so that updates after installation will decrease significantly.
EDIT:
But CentOS 7.3 hopefully will be released soon and in near future nethserver-iso will be based on that newer iso, so that updates after installation will decrease significantly.