iPhone 6 step counting

So I downloaded the new nike fuel app and I have my fitness pal app and the 24/7 app. I noticed all of them except the regular Heath app and 24/7 app have different step counts! Why is this? Which one is the accurate one? For example today The health app says I've walked 449 steps the Nike fuel app says 480 and the myfitness app says 596 steps. They are consistently different and are sometimes pretty far off from the other. The motionx 24/7 app pulls the data right from the health counter as it's the same. So what are these other apps doing? I have my height and weight entered into all of them. Maybe all of them interpret my height and weight differently and calculate my steps different? What do you guys think?