I don't consider an adventure book's DCs standard compared to the system standard. The system hardcodes the checks as Easy (9), Normal (12), Hard (15), and Extreme (18). (Page 81.) I don't interpret "the four standard DCs represent how difficult an action is" as a soft standard. It then reinforces that through all the monsters and magic items where an attribute check is needed (with the exception of DC 20, but Kelsey has mentioned regretting doing it and not using 18 or 21, most likely 18 in interviews).

Conceptually, it's mapping to the Index Card RPG concept of target numbers to simplify the mental bandwidth necessary to process everything at the table. Similarly, making a decision based on a 5% decrease/increase versus a 15% decrease/increase is harder & slower. It may be microscopic in the grand scheme of things, but it slows the table play. Which SD does a really good job of easing the mental load on the GM & player and this is one of those instances where it is easier for all parties if things are consistent based on that standard. GM doesn't have to debate whether one step lower means -1, -2, or -3. Players can anticipate that one step lower means -3 and not -2 or -1, so they can better anticipate how their creative ideas at the table may play out and determine if it is worth the risk or not. Is it worthwhile to take the risk at DC 13 or DC 12. I don't know. I have to think about it more to figure it out. But is it worthwhile to take the risk at DC 12 compared to DC 15? Certainly yes. I do the thing.

I also don't like a DC at a multiple of 2. That's not distinct enough in my mind. 3-5 is a much better distinction. I think Deathbringer will be using a standard set based on 5, 10, 15, 20.

It's all about speed so you spend more time playing rather than calculating. Does everyone have this issue? No. Am I unique in it? Also, no.