Google Analytics lies.

Based on my experience while creating userTrack.net, if you install 10 different analytics platforms you will get 10 different numbers of "users". This is because it's hard to define what a user is and also users could block that specific analytics script or simply can't run it.

The biggest issues that lead to different numbers:

• Adblockers - Google analytics is blocked by some adblockers
• What counts as a user/visit - If you use a DNS-only analytics platform (eg. Cloudflare Analytics), you will see as being a real user most of the crawlers/bots that don't even access your site, but only request the page source. So DNS-level analytics will show a much greater number of "users" than on-page analaytics.
• Brower specifications/settings - Fetch is not supported in all browsers, localStorage can be disabled for privacy reasons, some users even have their JavaScript disabled or only run JavaScript selectively for some domains.
• Bots/fake users - If you run ads on Facebook you may notice that most of the traffic they send is just fake users from click farms, which don't use your website and have a 0s session length. There is a reddit post I wrote about it a while ago.
• Latency (network speed) - If the analytics script takes too long to load (the user leaves before it loads), it won't track the visit anymore.

As you can see, those are just a few reasons why different analytics platforms could show different numbers. From my experience, the Google Analytics numbers are just an estimation, and are only good for estimating the magnitude of the traffic, not the actual number (eg. you know if you have 1, 100, 1k or 10k visitors), but the numbers in-between should be taken with a grain of salt, the biggest culprit being adblockers.

The title is totally misleading if you don't know what the actual reason is.
Also, this could have other reasons like browser support (https://caniuse.com/#feat=fetch)

I reread the post 3/4 times not sure what you're asking.