The way Page Speed Tests works vary from tool to tool and can often result in marginal differences.
For Example, GTMetrix will always try to run every script possible which is available on the page. What does that mean? For Example, There are some scripts from other domains which you have set to be run by the browser at the end since they usually don't contribute much to the user. The script can include the queries by some plugins like 'Flying Scripts' or etc. While Google Bots are smart enough to be able to differentiate them, Speed Test tools aren't smart enough and you should always consider that. Why would GTMetrix do so? So that You can monitor and check for every script your website is loading.
In Order to Check Your actual Website Loading Speeds, We Recommend Tools like Pingdom and dotcom.
Do note that, Server Test Location matters too and don't skip on it. If you are targeting the Indian Audience, check your Website Loading speeds Either from an Indian Server or Washington DC Based Server.