It doesn't have a '+' in front of the value in the Perception DC modifier column.For example, the "Not moving" entry has "+20", so you obviously need to add that 20 to something for it to make sense. The "Using Stealth" entry has "Stealth check +20", so there's no need to add it to anything, it's a base of its own.
If that entry was "+Stealth check +20" it would be clear it needed to be added to some other base value.
I see what you mean now. I didn't think of it as possibly meaning "re-setting the base DC" before you mentioned it. So with that line of reasoning, the stealth check becomes the new "base" with its a +20 adjustment, replacing the original 20 DC, which basically means just a -20 to how I was looking at it before?
Still, if that's the case why didn't they just put "+Stealth check" then? It's redundant to have to set a new base that starts at the same DC as the new base (EDIT: I meant old base), excluding the stealth check roll.