This isn't really confirmed at all as I don't have enough data to support my theory, but here goes:

Completion SP is a set number based on a few factors, some of which you can't really affect:
1) Stars
2) Speed of completion bonus
3) Guardian's Aspect
--Unlisted data support GA bonus as (endSP x 1.5)
4) Leve rank (Faction leves are exceptionally better)
5) ??? Some unknown modifier.
6) Your class rank imposes some kind of capped value.

The main components are the stars and speed bonus, with the rank-based ceiling.

Consider: All done as 45GLA
R40 (Broken Water) All Cracked Up
4star+GA: 5326

R40 (Broken Water) Thwack Ye Mole
3star+Speed+GA: 5326

R40 (Broken Water) A Devilet's Best Friend
4star+Speed+GA: 5326


R40 (Halitali) Juggling Knives
4star+speed+GA: 5858

R40 (Halitali) Bigger Fish to Fry
3star+speed+GA: 5858

R40 (Halitali) All Things Must Die
2star+speed+GA:4793

It's not especially obvious from these numbers, but from a lot of testing R30 leves (that I can't remember exact numbers for) I do know two things to be mostly true:
1) Speed bonus is equal to +1 star in terms of SP gain, even if you're already at 5 stars.
2) Guardian's Aspect multiplies end SP by around 1.5

I don't have the full map, but all leves are in a kind of tiered system, with Speed adding +1 star. R40 leves offer at least 4k with GA, R30 offers at least 3k, R20 5 star ends at 2k, and the trend probably continues. It's a pretty appropriate level curve.

What I can't explain yet is the ceiling value on rank (GLA 46 now so I should be able to see a difference later) and the differing cap between Halitali and Broken Water. A shellmate told me on MRD that he was getting drastically lower numbers in Uldah camps than at Iron Lake. Makes me suspect there's a popularity modifier like Behest is now supposed to have, or a "own guild region" modifier. Food for thought. I plan to test/compare Hali and Iron Lake leves next chance I get.

The other possibility is that not all leves are created equal.