Was it maybe the difference between one-handed and two-handed weapons, ie. was the Venom-sword one-handed and the “normal” legendary two-handed? If that’s the case, the thrall may be “proficient” in two-handed weapons.
There are a few threads discussing this bug/feature/weird alien conspiracy regarding thralls and their preferred weapons, but the experience of many people, myself included, is that thralls who originally used two-handed weapons before capture fight better with two-handed weapons and suck at using one-handed weapons (or refuse to fight with them at all), and thralls who came with one-handed weapons have similar trouble with two-handed weapons.
You can find reports of my highly scientific experiments regarding thrall weapon use here: Thralls no longer doing full melee combos