At the end of the day, the translation is not idiomatic. An American would say "The ball landed out of bounds" or "out of play." That's the way Oliver )i.e. Antosch and Linn) would translated it. Their sentences are always useful and always translated into an idiomatic equivalent. That's important, because you learn that you don't say things in another language by translating literally. You have to learn actual usage. Duo should be helping with that.
The frustrating thing about these poor translations is that I never know when DL is going to force me to be literal and nonsensical or accept an or connotative translation. Then it goes to the other extreme and uses expressions that are more at home in a press release, e.g. "the book hits the bookstores."
Then it would be OK to say the ball fell off something - it falls down - it can't fall on the same level. I think that's why we're all getting hot under the collar! The ball can fall off the footballer's foot - it would normally roll (not fall) off the pitch, over the line, out of play etc. [A native Brit].
Given that it doesn't say "the ball fell from the field", it's entirely possible that Duo's sentence is said in the sense of it being away from the field in question that the ball's action of falling happened.
It wasn't on(to) the field in which the ball fell (from whatever higher base/point it came from - such as a stadium roof, or the sky), it was off it.