My stolen Extensive modification to XP, skill ranks, feats, and max character level.

[Land Outcast]

It seems arbitrary that skill rank max would be 3 more than level,

Not Really,
A 1st level NPC trained in a field of expertise (4 ranks) can handle:

On A Hurry and Taking Chances:
Easy Tasks 100% of the time (DC 5)
Normal Tasks 75% of the time (DC 10)
Difficult Tasks 50% of the time (DC 15)
Complex Tasks 25% of the time (DC 20)

With Enough Time on his hands (taking 10):
Up to Normal Tasks: Always
(the normal working man manages the day-to-day in a satisfactory fashion if he is not on a rush)

Devoting All The Time Needed to Make it Perfect (taking 20):
Up to Complex Tasks: Always
Doesn't get to Excel. (DC 25)

Four Levels later, the peak of NPC skill (or so), the 5th level NPC (or PC) can manage complex tasks with just enough time on his hands (8 ranks, +3 Skill Focus), and will easily Excel if he devotes himself to the task.

This is the average man.

Heroes by definition are superior, therefore managing (more or less):
A 1st level PC trained in a field of expertise (4 ranks, +3 Ability, +3 Skill Focus)
can handle:
Easy Tasks 100% of the time (DC 5)
Normal Tasks 100% of the time (DC 10)
Difficult Tasks 75% of the time (DC 15)
Complex Tasks 50% of the time (DC 20)
Excel at Task 25% of the time (DC 25)

(4 ranks, +3 Ability, +3 Skill Focus)