Longtime UDP 100M test unstable

I tried iperf3.1.8 speed test between 2 Windows 10 PC connected through a switching HUB by the command

iperf3.exe -c --udp --port 5201 --reverse --interval 1 --time 18000 --omit 10 --bandwidth 100M --get-server-output --forceflush | Foreach{"{0}={1}" -f (Get-Date -Format 'yyyy/MM/dd-HH:mm:ss'), $_} | Tee-Object -FilePath "C:\XXX.txt"

iperf3.exe -s

Then,the Bitrate varies from 70Mbps to 100Mbps until about 11000 sec passed.
After about 11000 sec, Bitrate becomes stablized by 100Mbps.

So I tried iperf speed test between 2 Windows 10 PC connected with each other directly through LAN cable by the same command.
Then, the Bitrate was stablized up from 90Mbps.
However, as same as the first case, after about 11000 sec, Bitrate becomes stablized by 100Mbps.

Could anyone tell why 11000 sec could change the iperf result that clearly?