Please have a look at Best practices and tools for measuring wifi performance - #13 by Kimcha
"Where you run the testing tool server and client is important. [...]. For example, if you have a router with a weak CPU and run the iperf3 server on the router, then it's possible the speeds will be limited, because iperf3 itself uses up a large portion of the router's available CPU and starves the router of the CPU to achieve full wifi transmission rates." - Kimcha
In reverse roughly half the speed:
iperf3 -c 192.168.1.1 -P3 -t11 -R
[SUM] 0.00-11.01 sec 200 MBytes 152 Mbits/sec 238 sender
[SUM] 0.00-11.00 sec 197 MBytes 150 Mbits/sec receiver
This could also be explained by distance and walls. Your router might have for whatever reason lower signal sending power than hardware in your PC. As far as I am aware, by default iperf3 UPLOADS data from the client to the server (= data your PC sends). When using the reverse option, you measure the download (= data the router sends)