And it is indeed almost always true that the DC for successful casting is 10 + (2x spell level). What I noticed was that the 5th level Wizard spells all have 18 as their minimum successful casting DC while the 5th level Cleric spells are all DC 20.In general, your spell succeeds if your spell check is equal to or higher than a base DC of 10 + (2x spell level).
I don't think this is a big deal - especially since the text says "in general" (and, of course, ignoring and changing rules is encouraged, etc.) - but I noticed it and thought I should bring it up.