I agree. That is a problem in Garrosh's portrayal as his char. Even though he's been described as an incredible warrior, books such as RS gold the shattering hint that his father was one. However, it's never shown. He could have lost to Cairne, if not for the cheating, but lets be real Thrall could have taken on Cairne at the time. I believe that Garrosh is a strong fighter, but how much could he be according to your argument? Green Jesus isn't as great.
Thrall's upbringing and training. When we get to this point, Thrall hasn't seen actual fights in real life for years perhaps a decade or more. He was the one who led the uprising against the prison camps, formed the modern Horde after which he slipped into a leadership position, and later shifted into more of a shamanistic, primarily shamanistic job. There's no doubt that he has impressive muscle memory, and most likely, he got periodic sparring sessions or went for an outing with his PC, but you take an extended break from everything and OSRS buy gold you'll be a slug. He's certainly talented, but perhaps not as good as a more skilled or equally skilled warrior.