This is strange, but for whatever reasons the auto-complete feature in Nisus Pro doesn't work the way it's supposed to.
For example:
Type the phrase "Armenian bizarre" in Textedit. Now type "Arm" and activate auto-complete (escape key). The first word that pops up is Armenian. It know you have used this word and remembers it.
Now do the same in Nisus Writer Pro. The first suggestion is "Arm" followed by "Armada." This is far less helpful because NW is not recognizing the words you are actually using in the document.
The behavior of TextEdit does appear to be different than that of Nisus Writer Pro. I'll file a issue that we should do the same (eg: show words from the document in the auto complete list).