The Router limit the velocity of internet


I have a Router Peplink Balance 30 v4.8.1 build 1106.
I tested the velocity in Speedtest and through from modem, the download is 30mb. but through router the velocity is 18.
why is that?

I attached the test from speedtest.

Hello, you have a very old and original Classic Balance 30 that was only capable of a maximum 20Mbps throughtput. I recommend to update to the new version. Thanks.

Thank you Tim!