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.

I am guessing here, but it is an educated guess. I think the reason why the difference in flavour might be greater between different butters than between different brands of milk is that the fat content in butter is so much higher than it is in milk. Just think about meat: The main flavour component in any meat is the fat. If you take a lean cut of beef and compare its flavour to that of equally lean pork, chicken, duck etc. they will all taste quite similar. If you do the same comparison but with high-fat cuts the difference in flavour will be huge. Ergo, it should be the same for milk vs. butter, especially if you compare different brands of semi-skimmed or skimmed milk, where the natural fat content is reduced. But as I said, it is just an educated guess. Please correct me if I am wrong.

I agree with franko that the cow's diet will have the largest effect on the final flavor of the butter. This includes changes from season to season as the cows move from grazing outside on fresh grass to inside on hay during the winter.

In addition to that, some of the specialty butters such as Lurpak are cultured with bacteria that enhances the flavor.

Milk doesn't always taste the same, especially the organic brands which also have different nutritional values as well. It can taste quite different, but not completely. Also the ability to taste is subjective. Some can detect these differences while I suppose others cannot.

As to why the butter can taste different. The listed ingredients only make up part of the taste (those ingredients which have to be listed according to the FDA, not every ingredient is necessarily listed). A major factor in taste is how it is processed and preserved (preservation processes may occur more than once at different times). As food gets old it is going to lose flavor and/or taste different. To hide this there are many preservation techniques to mask this. Adding "natural flavor" (IE: highly concentrated artificial flavor derived from "natural" sources) to mask this is consider preservation and not required to be listed as an ingredient.

I'm going to guess that it's all based on the diet of the cows. Grass versus hay makes cheeses taste different, so I don't see why butters and milks should be any different.