The density issue really surprised me. Previously, I had thought the main difference was the flavor and the nutrient content. I'm guessing the raw milk butter has a stronger molecular structure because the fat globules have not been smashed with homogenization*, and the protein structure has not been damaged with pasteurization.
In the flavor category, if I make butter with fresh cream, it tastes similar to SBB but with more depth. T
* I have since thought this over and realized that there would be absolutely no sense in homogenizing the milk/cream in the process of making SBB. I checked my box for unsalted butter; the ingredients list said "pasteurized cream and other flavors".