It is about how you drive your car and I believe estimates a new full tank distance to empty based on your previous MPG calculation. So make sure you reset all the counters every time you fill up to get more accurate readings. I don't know if this is how they actually do it but this is what I've noticed. As for example if I got a low average MPG on the previous tank, when I fill up the DTE shows like 270 miles, but if I got a much higher average MPG then the new DTE shows something like 300 miles.
2016 Mazda CX-3 GT in Deep Crystal Blue on Parchment