One Question – Dice & Dragons - RPG Dice Roller Review

How are dice rolls determined? My concern is that it seems that this app rolls >13 quite often on d20. The other dice seem to be fine. I’d like to hear a dev response, in which case I will up to five stars. It’s a nice app for players and DMs. (This is not accusatory. I’m just curious and would like to feel good about the rolls.)
Review by Seventhfloordown on Dice & Dragons - RPG Dice Roller.