Why is the Bounce Rate in Custom Reports different to Bounce Rate in the Pages report?

Custom Reports with the dimensions « Page URL » or « Page Title » calculates the « Bounce Rate » differently to the Pages report.

Pages report Bounce Rate

The « Pages » report calculates the bounce rate for that page as the number of visits that started on that page (Entry page) and left the website without any further actions. As we can see from the tooltip in the « Pages » report:

Example:
if we have two visits as follow:
A visit that starts on /page1, goes to /page2, then exits.
A visit that starts on /page2 and immediately exits.

The « Pages » report will show the bounce rate for /page2 as 100% because all of the visits that started on that page immediately exited the website.

Custom report bounce rate

The « Custom Report » calculates the bounce rate for that page as the number of visits that had only a single page view in that visit including page views that started on other page URLs. As we can see in the bounce rate tooltip shown in the « Custom Report »:

Example (Using the same visits):
A visit that starts on /page1, goes to /page2, then exits.
A visit that starts on /page2 and immediately exits.

The « Custom Report » will show the bounce rate for /page2 as 50% because it will include the visit that navigated to /page2 from /page1.

At present it is not possible to change how the bounce rate is calculated in the Custom Reports, as Custom Reports do not have a metric for « Entry Visits ».