EXTREMELY simplistically, this comes down to leptin output. Leptin is generated by the fat you have. Within reason, leptin levels can trigger fat accumulation if there isn't much, and conversely as leptin increases fat burning increases (Invalid Link Removed). BUT it seems that the theory's only half baked, there's (recent) evidence that as fat levels accumulate, the body becomes "resistant" to the leptin signal and triggers other body changes (like making blood "clot" reducing energy efficiency: Invalid Link Removed).
Ergo, the chain breaks and you get fatter. Something like the theory of Syndrome X/insulin resistance.
So the modified (and highly tenous) theory says something like "The more fat, the more leptin, the more leptin, the more that the body starts to fight leptin, the less leptin does for you, finally excess calories go to fat." (note the magical word "theory" sprinkled liberally through here...)
From all evidence that I've seen and according to several guys I know who have bulked beginning at various fat percentages, you want to get very lean (under 10%) and then do a clean mass up . FYI when massing, sugar or other "fat storing" elements of your diet REALLY affect the amount of fat you put on. You're so heavy with food intake there is no room for crap in the diet...if you're going to "cheat" one day, one thing you can try is to cut the cals on your good carbs (while keeping protein the same) when bulking to "balance out".
Now, lean does not mean TOO lean. Testosterone and other hormones crash out and stall once fat levels get too low. So consider what food coming in does...
It appears there might be a "magical zone" in around 7-10% that's ideal.
I bulked before starting from 180 ~12% fat until I got to 195 at ~16%. I'm currently 168, 5.95% (all these according to 9-sites, I actually think they're underestimating by about 3%).
It is my intention to do a very clean increase as of today ("sculpting style") until 183 lbs, that should be a 15 lb increase either way. When it's over this round, I will compare the results. I anticipate it will take longer this time -- evidence that less fat's accumulating (assuming LBM gains at the same rate).
However, Gdawg's right -- how can I possibly rule out other factors (e.g. I now have a 1 year old daughter, stress, I'm a year older, etc. etc.) but it should be interesting. PM me on Invalid Link Removed in a couple of months for an update.