Guessing, could be wiring, could be your switch or even upstream.
Here I'm using LTE to connect and my numbers are really limited, RWIN seems to be stuck at 45056, MSS 1370 (changed to 536 after I changed MTU) and weirdest is MTU, 1410 usually but I just did a "ifconfig lan0 mtu 576" and now its stuck at 576 no matter if I try setting it to 1500, 1410, 1200, 600. Didn't try lower.
Here it is likely a limit of using LTE but could be the hub, which I'm cabled to. Hmm, trying on my phone (WiFi) gives the same default numbers (before I played with MTU on the desktop) as I had. Also mentions that Chrome can change the numbers independently of the OS on sockets it creates on some OSes.