FitBit vs Cronometer calories burnt don't correspond

I'm having a bit of a problem with the syncing of data with FitBit. The data transfers but the calorie count vastly differers. I've got it on the setting of no activity as I'm using the FitBit and the minutes of excersice corresponts with both apps, but 30 mins of walking would add 100 calories burnt on the FitBit app (reasonable) and then nearly 1000 calories on the Cronometer app (vastly overestimating). How can I adjust this? as this obviously makes using the app less usefull if it's going to give rediculous numbers.