steam oven vs conventional oven
As you may already know, meat becomes tender when cooking because collagen (which is chewy) breaks down into gelatin (which isn't chewy), and the longer it cooks, the more collagen is broken down.

Having said that, if the bag in the conventional oven was in for closer to 4h and the bag in the steam oven was in for closer to 6h, it's kind of obvious why the steam oven meat was more tender (50% more time does make quite a bit of difference!).

If the bags were in the oven for the about the same time, the steam oven bag probably cooked longer anyway as steam transfers heat more effectively than dry air - by the time the meat in the conventional oven reached 180F, the meat in the steam oven had already been sitting at that temperature there for a while, and so, more collagen had time to break down. Steam should also provide a more even heat, with less fluctuations (having greater thermal capacity).

Lastly, it could just have been that the meats were a bit different - the tougher from an older animal, a different breed, or one of a thousand other reasons.