I don't work for Paizo, but I did read about the setting of
Pathfinder over on their forums.
Basically, they've sketched out a really large world, and all
Pathfinder and GameMastery adventures will take place within it. We can therefore assume that variety in tone and setting will come from exploring different areas of the evolving setting.
If you look at the descriptions of the first six issues, there are several different kinds of articles supplementing the adventures:
- A description of the town of Sandpoint
- Details on the fallen empire of Thassilon
- A description of the city of Magnimar
- Details on Desna, the goddess of travel and dreams
- Rules for maintaining and running a castle
- A description of the wilderness region in which Rise of the Runelords takes place
- Information on the stone giants of the region
- Details on the goddess of monsters
- Information on the dragons of the region
- Details of the lost sin magic of Thassilon
- A description of the ruined city of Xin-Shalast
- Information on the villainous mastermind of the Adventure Path
- Roughly a half-dozen new monsters in every issue, which either appear in the adventure or are thematically appropriate to be added to it
As James Jacobs put it, each adventure runs to roughly 50 pages, leaving 46 more for supplementary material of this type.
What I don't see are things like prestige classes, feats, spells, and so on, but there's
plenty of room for that sort of thing in the supplementary articles. A fifteen-page article on stone giants could easily present giant-related feats and prestige classes, both for the giants themselves and their enemies.