Well, its just that according to DnD 5e rules, you only crit if you roll a NATURAL 20, so without any modifiers. thats a 5% chance if you neither have disadvantage or advantage.
Yeah I do not know the system well enough, so I was mostly talking out of my arse here. I just figured through skillchecks that there are modifiers working both ways that are not always transparent to the player.
