You are taking two separate issues (DC to notice invisible creature, and invisible bonus to stealth) and merging them together when they are completely unrelated.
The bonus to stealth checks is a bonus. It applies to stealth checks. It doesn't apply when you're not making a stealth check.
The DC to notice an invisible creature is 20. This is not including the bonus to stealth checks, because it applies even if the creature is not using stealth.
For the record, I'm with Grick on this issue. The "extra" +20 is from the bonus for being invisible under the stealth skill (and would be a +40 if the stealthing invisible creature was stationary).
Its entirely separate from modifiers to perception DC checks derived from being invisible.
IMHO of course.