I think strain plays a big part, but beyond that lighting can/does play a big part. I've seen it first hand, tried a couple of runs with some cheap LED's, gave up on them, switched to CMH, same strain, clones from the same moms I had run under the cheap LED's and tripled the weight/density. Nutes/medium also has an effect. I recently finished a run with a strain I had run before (same environment) but was prone to burn (nute and light burn) so on the last run I backed off the nutes and while it produced a lot of bud, they were like 2/3 the density as previous runs. So, imo there are multiple reasons for fluffy buds, genetics have to be there, but from my observations nutes/feed/medium and lighting can play a big role and do affect density.