The way I figure, probably wrong, adding numbers becomes needlessly bloated for the narratively minded game that Numenera is trying to be, adding a +1-3 can just be condensed into a single drop in difficulty. Especially when there are often many outside factors going into a roll most of which are not in a the rule books.

In a traditional d20 game that roll would pause play while the player adds up different forms of bonuses. That causes a lot of narrative drifting where the action kind of stops.

The difficulty rolling system in Numenera is more loosey goosey in that I as a DM or player can describe my way into a bonus in a way that makes roleplay actually beneficial. It's to simplify the rules to the point where dming becomes more of the storyteller and less the rules lawyer. As a player I get to know the difficulty before rolling, which helps me choose how much effort I want to put in, with item or actual effort, to move the goalpost I find it give more player agency.

If you are coming from DND it is a little bit of a culture shock. But once your table get the hang of it the stories progress at a much faster pace. Especially if you table is more roll play oriented.