You likely have a character that already explored large portion of the map from before you installed ZenMap or you ran the command exploremap to reveal the full map and never reset it.

You can remove all your explored sections of the map with the vanilla console commands:

resetmap and resetsharedmap

Caution! after you run these commands your character will have the map wiped clean. This doesn't matter if you are using ZenMap as the character's exploration radius is not used. However if you plan to use your character on the same world without ZenMap then you probably don't want to reset that character's explored area.