The A.I., as the difficulty suggests, changes, so someone has the choice to beat all 10 presets within the same difficulty. So each difficulty is separated as a subcategory for people who prefer a certain difficulty to beat each preset; similar to the Classic Mode speedrun of Super Smash Bros. Melee where people are given the option to submit a run under one of the listed difficulties. People aren't just going to pick a random difficulty for each preset as that would be pointless. It's better to separate it with each difficulty as the A.I. gets harder. What doesn't effect anything in the game, however, is the time to complete the night. Just like Night 4, the window of time to complete the night is fixed since the point is to survive. The fastest times under Custom Night would involve people selecting the preset fast enough as well as completing each preset without dying. That's similar to the other FNaF games where the amount of time to complete a night is fixed.