Why is my encumburance +10?

All my gear fits me perfectly, and for some reason I still have +10 on my torso. Is it my vine limbs mutation?

Yes, the Vine Limbs mutation adds a flat +10 to your torso encumberance, same as the Lower Level of the mutation simply called ‘Vines’ .
If you want to get rid of that encumberance, either get rid of the vine mutation, OR reach the next level, which is ‘Vine Sprouter’ .