The web page visit is partially tracked only if users fill out a form otherwise it goes unnoticed.
In several tests the Activity Log shows "Fill out a form" without a page visit. From the traditional standpoint that could be an obscure bug. How users could fill out a form without visiting the page?
Google Chrome introduced Incognito. Firefox, Safari, Opera and Internet Explorer implemented as Private Browsing.
Pages users visit in Private Browsing/Incognito will not leave traces, like cookies, on users' computers after they close all open windows.
A normal visit to any page is not tracked during visits, regardless the navigation path. That includes other pages after the form is filled out.
Private browsing is a relatively new feature. It is a new challenge to tracking, applicable to any software and company evaluating users activity.
The new Do Not Track feature is a contributing factor for apparently incomplete Activity Logs.
When users turn on the Do-not-track feature, the browser tells every website they visit (as well as their advertisers and other content providers) that they do not want their browsing behavior tracked. Honoring this setting is voluntary — individual websites are not required to respect it. Websites that do honor this setting should automatically stop tracking your behavior without any further action from users.