If you are deep in the weeds of Core Web Vitals, checking and analyzing scores across multiple pages, then we are 99.9% sure that a certain question has persisted in your minds.
That question is this: why are Core Web Vitals scores constantly changing?
Luckily, Google’s very own SEO guru John Mueller was recently at hand to answer the questions as to why scores seem to always change despite webpages remaining the same.
Before we talk about the SEO office hours session on Core Web Vitals where John was talking, let’s first lay out what Core Web Vitals scores are. 
There are two kinds of Core Web Vitals scores: field data and lab data.
Field data is recorded from the real-life visitors that are coming to a webpage in the real world. The reason for field data is to display the real-world user experience of real site visitors to a given webpage.
Lab data is generated by simulated visits, i.e., a bot from Google Lighthouse that uses a user agent that visits the page that is tested, and an algorithm simulates the visit on a mobile as well as on a throttled internet connection. This data is then collected and a Core Web Vitals score is generated.
Google Lighthouse is “an open-source, automated tool for improving the quality of web pages,” according to its official site. “It has audits for performance, accessibility, progressive web apps, SEO and more.
“You can run Lighthouse in Chrome DevTools, from the command line, or as a Node module. You give Lighthouse a URL to audit, it runs a series of audits against the page, and then it generates a report on how well the page did.
“From there, use the failing audits as indicators on how to improve the page. Each audit has a reference doc explaining why the audit is important, as well as how to fix it.”
Back to the question
So, why do Core Web Vitals scores change despite webpages remaining the same?
John Mueller answered the question from the point of view of field data – real-world users.
More specifically, the question asked was this: “When does the Core Web Vitals give steady and correct information? It keeps changing without changing any data on the website.”
John’s answer pointed toward random factors having an influence. “So, I think this is probably a side effect of how the Core Web Vitals and the page experience update is processed. 
“And that’s something where I would try to look up those details to understand a little bit more about how the field data, the data that users actually see, kind of plays a role into this.
“And that is something where if users from a wide variety of backgrounds and different locations and different device types access your pages, you will probably see some fluctuations over time there as well.”
Varying internet velocity
A fast server and website are only parts of getting a high Core Web Vitals score and it’s the only part that’s under the control of the SEO or site publisher.
Echoing John Mueller, there are a wide variety of factors that influence the scores of Core Web Vitals and many of those factors cannot always be controlled.
Even when you have a website hosted on a fast server, factors such as network congestion, an outdated cell phone used by a site visitor, and a poor mobile data connection can all significantly impact the performance of Core Web Vitals.
There there’s the possibility of some site visitors being on a newer model phone that has a powerful processor operating on 5G. That site visitor is going to generate some pretty high Core Web Vitals scores when it comes to field data.
Then there are issues such as network latency that can affect both the real-world field data and the simulated lab data. So despite lab data not representing a real visitor, a Chrome browser-based bot visits the specific webpage that is being tested. And this is also going to affect the simulated lab data scores.
PS: ArganoUV is one of the world’s leading Core Web Vitals development teams. Contact us to see how we can work together.
