It does appear to be based on K/D. Comparing mine to OrdealByFire's our SPM is very close but his K/D is much higher than mine.
He's at 197 and I'm at -104. I play only Conquest and go for the flags with little concern for my K/D and apparently the game punishes you for playing objective modes and actually completing the objectives.
Jeah the skill calculation is wierd to say the least. SPM is a more determining factor imho. A team of high spm players versus a team of high skill players will see the SPM players take the win easy as they go for the objectives without regards for their
lives. BC1 is a clear example of this, often a team consisting of all level 8-14 players is way more dangerous and succesfull then a team of 2's.
In my book Skill should be a calculation factoring in; win/loss ratio, spm, k/d ratio, hit/miss ratio at an equal rate. BC1 wise spm did seem to matter a little bit (roughly 20%) with 30% being determined by the number of kills and 50% by the number of deaths
(numbers based on feeling).