RT-ACRH13 WAN Speed 100 But Negotiated 1000

Anybody know how it's possible that 1000 Full-duplex is negotiated, but the speed won't go past 100?
I get almost exactly 12MB/s max. If I connect the same routers together with wireless client, I get around 40MB/s from the same computers, so there's something definitely strange. I don't have a long enough cable to try another for now.

I was wondering though, how it's possible to negotiate 1000 but can't get more than 100?
I go to Switch in Luci and it shows 1000