from what im gathering basically D4C can bring forth other parallel universes/scenarios into the universe it's presently in, causing them to collide with one another. So in one universe, Dio shot Johnny. In another universe, Wekapipo shot Johnny, and then in one universe Valentine shot Johnny. The stand has the ability to literally bring all of those scenarios/parallel universes into the same realm at the same time. Which is why the painter and the children saw different things and all of them were telling the truth.
Here's an example I can think of to explain it a little better: For those of you who know Doctor Strange's ability from Marvel, and how he can see all the different possibilities of a situation/get info from parallel universes. Now imagine all those different scenarios, but literally physically happening at the same time in the same place. That's kind of what D4C can do.