I think it's just two different words that mean the same thing: the air inside the incubator is being moved by a fan. "Circulated" seems like a better word choice to me, but "forced" means the same. Both words are used to indicate that it's not a "still air" incubator, but you probably knew that.
I thought that it was probably the same thing. I guess just different wording for each. They are on the same website and they look the same. The "forced air" was more than the "circulated air" one.