In the 4th edition DMGs (DMG p42, DMG2 p80), recommended DCs for easy, moderate, and hard situations are provided in tables. What's interesting is that the levels in the left-hand column are grouped by 3's.
I thought maybe this had to do with 1/2 level modifier, but that goes up every two levels. I thought maybe it had to do with increasing ability scores, but those go up every four levels. It seems like the number 3 is arbitrary here.
What is the motivation for putting DCs in groups of 3's?