OK, what aircraft ? 737NGs I assume?
It does have to do with:
1. Their speed/cost index policy
2. Their 1st choice altitude policy
4. Holding & Alternate policy
That, gives you the fuel burn in kg/km...
Now, then we go to:
5. Where and who do they buy fuel from? (vendor pricing policy)
6. Do they conduct tankering (this reflects on effects of #3 and #4)
(and the list goes on)
If this is not aircraft type specific, THY's diverse fleet would give that positive correlation instead of a negative one, as with widebodies, the difference in fuel burn per hour changes a lot throughout the whole flyable grossweight change... etc etc etc...
When losing situational awareness, pray Cumulus Granitus isn't nearby !