The study by the University of Reading is doing the reflecting upon the soft drink/tooth decay find.
the Moon reflects light from the Sun
hmm, these two sentences are a little different. First of all, when you add the preposition 'upon' (I think its a preposition I may be wrong) you're changing the direction of the verb completely. To reflect 'on' or 'upon' something is different from something being reflected 'in' something else. Examples:
'After the fight she reflected on his words and became even more upset.'
'I sat by the lake and reflected upon the days events.'
The rough translation of 'reflect on/upon' is 'to think about', 'to ponder', 'to consider heavily', etc. Your example makes the U of Reading study a subject, which is thinking about the new information on tooth decay.
If you want one thing to directly mirror or conform to another thing, the particle 'in' must be used, or the verb must be used without any prepositions. Your second example about the moon is correct. Another way to say it with the word 'by' is:
'The sunlight is reflected by the moon.'
To return to your first example, you could get away with saying:
'The findings on tooth decay are reflected by a study done by the University of Reading.' In my mind this sentence makes the study come first, and the new findings are secondary information which confirm the original study. This has a slightly longer timeline to it, such that the author might be using one item to give strength to the other en route to some conclusion.
Your originally quoted example:
'This (the findings on tooth decay) reflects a study done by the University of Reading.' This sentence, to me, means that the two results resemble one another closely and may have even been done around the same time period. The author is trying to emphasize a point, these two pieces of research are pointing to that same result, with both being of equal value.