Relationship between session, "pageview ID" and page_ping event

I have enabled the webpage context to have the “pageview ID”, and also enabled page_ping event to measure that pageview’s total time. However, I have some pageviews that have page_pings across multiple sessions - is this normal?

@hoanghapham, check what the corresponding user and device is, for example, by examining useragent. It is not likely to be a bot due to a nature of the event (here, page_ping). However, it could be a lightweight browser such as Google WebLight, for example. This kind of browsers could be found on mobile devices. These browsers render webpages optimized for low-speed internet connection and result in stripping off some “unnecessary elements from the page” such as Javascript libraries which prevents generating a unique event ID.