What's Happening With Internet Speed?

Question

Computer networks

I have some kind of magic going on with the speed of the Internet.
So, my ISP advertised 10 Mbps (12, 14 subsequent months, but that doesn't matter). At night, the speed is doubled (20 Mbps). And it all works if you connect the cable directly to the computer.
But now I connect Wi-Fi , and the first day went, speed 10. Night, speed 20.
The next day, speed 20. Yes, yes!
I connect the cable directly, speed 10. Through Wi-Fi, speed 20.
The money is over. If you pay on the same day (at 7:00 deduction of money), then everything will continue. But if you pay the next day , everything is reset.
And on the first day through wi-fi, and directly speed 10.
After the night (the next day), the speed remains at night, but only through Wi-Fi.
This has been going on for about a year now.
I'm not a zero in the networks, but this defies my logical explanation ...
Once this went away for a month, then it appeared again.
I check the speed on real downloads (from torrent and not only) and on Speedtest.net

Answer 1

A bug in the network configuration by the provider
A bug in the billing
Incentive program Find out the
necessary from the provider.

Answer 2

Tariff speed. Usually, a normal provider uses the bandwidth with a margin for traffic bursts, for example, in the evening.