Originally Posted by Alon Binyamin
I wonder if the game actually rolls two dice in the background or if all the chances are just pre populated in a table and the game choose the correct one to apply.

For example: for a target of 10 a single roll has 55% to win, advantage has 79.75% to win, and disadvantage has 30.25% to win. The game only need to apply the correct chance to display on the screen without actually rolling anything in the background.

Interesting idea but I doubt the game is doing this. After all, the game does show the actual d20 result in the combat log/dialogue window.
It would be needlessly complex to do what you're suggesting (convert all d20 rolls, including adv/disadvantage, to flat percentages) but then convert the percentage back to the corresponding # on a d20 (which only increments in percentages of 5%, so 79.75% would have to be rounded to a 5 or 6 on the die).

Originally Posted by dreambled
I see what's happening here. I am using reroll in the literal sense of the action, not in the sense meant to explain how Advantage/Disadvantage works, that's my bad and I can see why you would think I believe that. In my D&D game I just have 1d20 to roll, so when I have Advantage/Disadvantage I have to reroll the same dice to get the other number. In this game, we have 1d20 on the screen, so iIf we have Advantage/Disadvantage, we should be able to use the Reroll button in order to get the second number. If Larian wants to get fancy, then maybe in a future update they will have two d20s on the screen when you have Advantage/Disadvantage.

Yep, what @LukasPrism says. There is absolutely no reason to not roll twice (your "reroll"), so the game should just do it automatically. Also this preserves other game mechanics which actually allow rerolls (e.g., Halflings can reroll 1's)