Why do diff cuts of beef taste differently?
What is it that 'causes' flavour in different cuts of beef, it's not just the fat levels is it?
Why would a bavette taste almost liver-ish but a onglet does not, though their fat levels are quite comparable?
Or comparing bone-in cuts like beef shanks which have a rich flavor that is very different from the flavour of a bone-in ribeye.
(comparing cuts from the same animal, same with the same aging, of course)
Think of it in terms of different aspects of the body. The liver is a filter, but there's also muscle, organs, fatty and not-so-fatty tissue, etc. Function changes things in just about anything. I wouldn't expect a cow's tongue to taste the same as its hind quarters. The function and location on the body of any given animal will change its structure and consistency, not to mention its texture.
Does that make it any clearer?
Different muscles get different amounts of exercise. The more exercised muscles are tougher but more flavorful, whereas the less exercised are tenderer but less flavorful. That explains some of the difference.
It's not just the amount of fat (or as you say, "fat levels"). It's, more importantly, the distribution of fat throughout each cut of beef -- even though the "fat level" may be the same for both cuts.
Also, because different parts of the animal receive different amounts of movement, or exercise, the tenderness of the meat will vary. Consider that both top sirloin and a filet will both be quite lean, but the former will be generally tougher than the latter.
Certainly it has to do with fat content and how much use the muscle gets but when I make a flat iron steak, which comes from the top blade shoulder roast the flavor is very different than other steaks like flank, sirloin or rib eye. They all have a similar beefy taste but differ in texture due to fat content. The shoulder just taste really different.