Board Game Arena

Hey guys just wondering how the K factor is determined during ELO calculation. I saw that during a game mu opponent and I were in a same range ~100 and ~150 but the K factors were different 20 and 40, which resulted in him losing 25 points and me gaining just 10 points.
One would assume, that the loss and gains are equal,but it's not, so what's happening here?

Thanks!!