My suspicion here is that black tea, especially when prepared in the British style, has more caffeine per cup because it's brewed to be much stronger than green

My suspicion here is that black tea, especially when prepared in the British style, has more caffeine per cup because it's brewed to be much stronger than green teas typically are. So both can be true, black tea typically has more caffeine per cup, but black tea leaves have less caffeine than green tea leaves when compared by weight or volume.