Why do different brands of butter taste different?
Cook's Illustrated (AKA America's Test Kitchen and Cook's Country) has done taste tests of various brands of butter, salted and unsalted, cultured and not cultured.

They found that the single most important thing in unsalted butter was how it was wrapped. Butter wrapped in foil doesn't pick up off flavors from its environment. Land O Lakes (incidentally my go-to brand) treats the parchment it wraps its butter in, and that parchment does keep out off flavors. Butter wrapped in regular parchment not only picks up off flavors, but over time loses moisture. The wrapping is an issue in salted butter as well, but not as big of one.

Secondly, obviously some butters are cultured and others are not. In the United States, most cultured butter is imported and is significantly more expensive. In the tastings, most people preferred cultured butter to uncultured when used as a spread. When used in baking, they found no difference. 

The third issue they found was whether or not the cows were grass fed. Some tasters picked up what they called "barnyard notes" in the grass fed. Some tasters liked that, others did not.

In salted butter, the amount of salt varies widely. For use as a spread, most tasters preferred brands with more salt. For baking and cooking they almost always recommend unsalted butter.

Just FYI, Cook's Illustrated recommends keeping butter in the freezer until just before the stick's first use. Even foil wrapped butter will pick up some off flavors from long storage in the refrigerator.