The Visibility Score refers to the keywords that you monitor in the ranking monitor.
If you check 5 keywords and your website is ranked on position 1 for all of these keywords on all checked search engines, you will get a Visibility Score of 100%.
If you add 5 additional keywords for which your website is not ranked, your Visibility Score will decrease although you are still ranked on position 1 for your first keywords.
How the Visibility Score is calculated:
The Visibility Score changes if the rankings change and/if you add or remove keywords in the Ranking Monitor.