Zero unique pageviews with secondary dimensions

Question

We have Google Analytics set up through Google Tag Manager on our SPA website. We are sending virtual pageview calls on every page through a custom event in a dataLayer object. And not using the standard pageview triggers that are available in the tag manager. In the same pageview hit we are sending many custom dimensions relevant to the webpage. In the all pages report when we look at the pageview data, without any secondary dimension then the pageviews and unique pageviews numbers are expected. However, when I apply a secondary dimension I see many unique pageview numbers turn to ZERO. This error is consistent with secondary dimensions that I send in the pageview hit (like time of the hit). And also with some of out of the box dimensions (like hour index). Without custom dimensions:

With custom dimensions sent in the pageview hit:

With hour index dimensions:

The total numbers are not changing with the addition of custom dimensions. I'm not sure why this is happening. Can anybody please help me understand the reason behind this?

Answer 1

This happens beacuse the "unique" count of pageview (or event) is associated with the first page on which it occurred, so for the second or subsequent page where the same page (or event) arose, since it is no longer unique for the current session, its value will be 0.

You can find here a comprehensive event-based explanation of this concept, but the concept is the same for page views: https://www.analyticstraps.com/eventi-totali-senza-eventi-unici/