The probability in a free hit depends on how aggressive the batsman is at a moment. While the aggression is set to the highest level, if a batsman was very defensive going into the free hit, they're going to have a harder time launching one into the crowd, for example.
Also, from what I've noticed in real life, most free hits end up as just a 1 or 2, rarely do we see a massive hit, because the batsman is just usually trying to hit it too hard.