The Puzzle of Creating Best Sellers

Trying to judge which books will catch the public imagination doesn't get any easier, however long you struggle on in the publishing world.
I was pretty certain as I wrote "Cry Myself to Sleep" with Joe Peters that it was a story that people would really want to read, but follow-up books are always hard to pull off. We had had a number one success with "Cry Silent Tears" and it seemed almost too much to hope we could do it again, however much Joe might deserve it.
The second book picks up his story from when he ran away to London at sixteen to live on the streets. It is all about what happens to damaged kids once they grow up and go looking for a place in the world. Joe is an incredibly inspiring character.
Thankfully, my gut feeling has been proved right and the book is already at number three in the charts, battling it out with Jade and Barak and the rest. But then I was equally sure that "For the Love of Julie" which I wrote for Ann Ming, would soar straight to the top of the charts. Everyone who reads this story of a mother fighting for justice for her murdered daughter, (whose body she herself found,months after the killing), tells me that they sobbed almost all the way through it. Ann is another inspiring person who has achieved the most incredible things in getting the 800 year old law of double jeopardy changed, but the book has not soared as fast as Joe's.
I'm confident that word of mouth will result in big sales for Ann eventually, albeit at a slower and steadier rate, but it is a puzzle why the one title has taken off so much faster than the other.
In between these two lies "Disgraced", which I wrote for Saira Ahmed, telling of how she ended up on the game after escaping from her arranged marriage. The book went into the charts, and is selling well around the world, but did not go straight into the top three like Joe.
I guess if we could predict accurately who the big winners were going to be, some of the excitement would have gone.