"Bitterness of tea is generally ascribed to the combination of catechins, saponin, caffeine, and amino acids. Depending on molecular weight, catechins can be bitter or astringent, whereas saponins are often described as acrid"
American Journal of Clinical Nutrition
On that same page they also state that "The biology of bitter-taste perception is poorly understood. The long-term challenge has been to explain how so many structurally unrelated compounds can give rise to a uniform bitter taste."
The only thing I didn't like about the article was that they often mixed acrid, bitter and astringent without specifically saying which compound does what.
Caffeine for example, is never said to be astringent in any online source, while catechins are often referred to being astringent. However it appears that astringency increases the perceived bitternes along with other compounds (even if they aren't bitter or astringent).
In this link:http://www.coffeeresearch.org/science/bittermain.htm, there are a lot of reasons that affect the perceived bitterness of coffee. For example, "Coffee bitterness is decreased by the addition of sucrose, sodium chloride, or citric acid. Hydrocolloids, in general, were found to decrease the perception of coffee bitterness (Pangborn, 161)."
In conclusion, AdamMY was right from the beginning, it's way more complicated than just a matter of amounts of specific compounds like catechins or caffeine.