Why does the network lose speed?

Question

Computer networks

There is the following network structure:
- VDSL router, to one of the ports of which computer-A is connected, and to the other one Powerline adapter.
- computer-B is connected to another Powerline adapter.
Speed ​​problem.
1) When I run speedtest on computer-A, I get 32 ​​megabits strictly given by the provider.
2) When I run speedtest on computer-B I get something floating between 21 and 29 megabits, usually around 23-25.
3) When I check the connection speed between A and B using iperf - it consistently passes 42 megabits.
Speedtest checks are run every time against the same server, the measurements were taken many times at different times and are always the same on average.
Powerline adapters -www.tp-link.com/en/products/details/cat-18_TL-PA40...
It is not clear why, with a network bandwidth of at least 42 megabits, the incoming 32 megabits from the provider are cut off to 20. What is floating.

Answer 1

Because tcp packet test or udp test are completely different things.
For the purity of the experiment, measure with external iperf servers .