A shot at the why, would be that his digestion system might handle lemon better than chocolate, in combination with aquired taste, like the good memory of eating the first lemon cake at his birthday party and so on.

But objectivly proofing it, would require a very complex bio-chemical anaylsis of his digestion and neurological system as well as a deep psychoanalysis. Interesting research probably, but not necessary for determining that objectivly lemon cake has a higher value to him, because he simply likes it more.