It is well known, that fighters can fight with many weapons, but as you can imagine, each fighter has his favourite one. As you are fighting with exact weapon and I mean exact, not same type or name, your skills in fight with this weapon are improving, but fight skills with every other weapon are decreasing. In fact this skill allows you to grow together with your weapon enhancing your abilities to hit and damage enemy, and also your parrying ability as well. Using command knownweapons will show you which weapons you are familiar with and will let you know if you have them equipped or in inventory at the moment.