Its how the hieharchy works.
You have the core books. Player's Handbook, Dungeon Master's Guide, and Monster Manual. Expanded Psionics Handbook would be considered a core book if you plan on using Psionics. All core books override any side books, like the Complete Series, if there is a discrepency.
Any campaign specific books override the core and side books, if there is any discepency, for that particular setting.