The completions provided by plugins aren't persistent.
For example try this plugin:
import sublime, sublime_plugin
self.toggle = False
def on_query_completions(self, view, prefix, locations):
self.toggle = not self.toggle
return ("something", "something")]
Open up a new text file and enter:
Then on the next line hit ctrl+space, then esc, then ctrl+space, then esc, then ctrl+space and you'll see that "something" only shows up in the completion list every other time as provided by the plugin.
If stuff shows up in the list it's either because the built in auto complete sees that word somewhere in your file or because your plugin keeps returning the same list of completions.