We use the sword of Crom not the thralls. This was the intention of devs, that’s why…

Sword of Crom has become an amazing role play weapon and it provides you extra bonus by using it, that not visible on your stats. If you want to test it your own go 100% encumbrance and equip it. You’ll get the Xtra strength bonus, your encumbrance will go 99% but nothing will change on your stats panel.