Hi Samir, my name is Neil, and I'm a Windows 10 user like you.
The speed all depends on a number of factors, even more so over WiFi.
My own connection is fibre and I am using Ethernet for connection direct to my router. My speed test results are below. I have put this there to show you how speeds can alter, even for a connection like mine.
First of all, all because you have a fast connection, doesn't mean you can download at the top speed all the time. Where you are downloading from will have a limit on how fast they can send the information from their server to your computer through your connection.
This might be top speed sometimes, but if other users are downloading from the same place, this may limit your speed. Hence why you can get wildly different results downloading from the same source.
If I download a game from Xbox on my PC, my top speed is usually around 500Mbps..which is about half my top speed. However, sometimes this drops to a mere 3Mbps when the service is very busy.
Downloading from different websites has dramatically different speeds. Sometimes I can get almost top speed, other times just 40Mbps. Depends where I am downloading from.
Sometimes my house is full of my daughters and their boyfriends and they're all streaming or gaming and this impacts my download speed.
Now you're on WiFi. This has added complications with speed due to other factors. Obviously you'll have the same issues as me above, but you can also get interference from outside sources that affect your speed dramatically. A cordless telephone, another wireless device, next doors WiFi doorbell that interferes with your WiFi signal and slow down the speed, as your router has to 'untangle' the signal from all the other wireless signals passing through it.
Also, the distance you are from your router, walls in-between you and the router, people walking by with mobile phones in their pockets, everything has an impact on WiFi speeds.
WiFi is generally a lot slower than Ethernet. Ethernet is the best you will get and is protected from outside interference, but this does tie you down with a cable to your device. Not a problem for a static desktop, but can be for a laptop.
However, if you were to try an Ethernet cable you will find A LOT more consistency with your speed than you would with WiFi. WiFi is just susceptible to interference.
I hope this explains why sometimes your speed can be dramatically different sometimes when you test it.
If you have any further questions, please don't hesitate to ask.
kindest regards,
Neil