Situation: Asus router RT-AC88U (fiber connected, 500Mbps)
Tablet Samsung Galaxy Tab S 10.5 WiFi

Using 2 different apps
Tablet is just 1 meter away from the router
Ssignal is strong

Run each app twice, immediately one after the other.

Below are the results.

I used the Ookla app and some other 'high rated' app.
Maybe there are better apps available, or better ways to test.

But I wonder why the speed is so fluctuating.

The 'wired' speed (PC) is a steady 'straight line' 520/525Mbps up- and download.