Different testing parameters and testing server environments are why. See https://gtmetrix.com/blog/why-is-my-gtmetrix-performance-score-different-from-pagespeed-insights-webpagetest-and-web-dev/
In general, all tools have differences in hardware, connection speed, locations, screen resolutions, and test methodology.
GTmetrix uses a real browser to load your page with an Unthrottled Connection using our specific hardware and test options. Some of these options (location, connection speed, screen resolution, etc.) can be changed based on your requirements.
WebPageTest also uses a real browser to load your page, albeit with different default test options and configurations. Some of these options can also be changed depending on your requirements.
PageSpeed Insights and web.dev launch performance tests using headless/emulated browsers and use network throttling to simulate your page load under different network conditions (i.e., desktop and mobile). You cannot change the test options like location, connection speed, screen resolution, etc.
Moreover, the Google tools (PageSpeed Insights and web.dev) use Lantern , which results in significantly different page loading behaviours, yielding different Performance scores and test results.