I think it could be argued that generally, legendaries are more likely to be posting better content, but without going through each post, that would likely be hard to accurately predict.
Apart from the fact that legendaries are more likely to be good content posters, let us consider this;
The merit sources who also are more of legendaries have been here for a long time. They have been meeting one another here for many years, apart from the content of the posts, they have falling inlove with the writing patterns of one another, their tones, how humorous, commanding or entertaining they will be. So, meriting such posts becomes a habit, most times without paying attention to the contents.
That is adaptation, a strong human character you cannot undermined. For the newbies or low ranking members to have such a share, they must last longer here and develop a unique writing style lovable by many.
Secondly, the law of reciprocation is indirectly playing out. Merit seems to flow to those who has the ability and enablement to reciprocate. A Merit source can easily give out 50merits to a legendary fellow, knowing too well that it is cyclical, re-genaratable and reciprocal.
Finally, I believe merit sources do not give higher merits to lower ranking members because they believe they may abuse the smerits they will have as a result of the given merits. It's somewhat showing that the merit system works in conjunction with the trust system. If you are trusted, you are trusted with merits too. Knowing too well you may not play the hide and seek game with your alt on merit issuance.
I am of the opinion that lower ranking member(s) who is able to produce a few quality posts, can be trusted and has a decent history of smerit allocation should benefit evenly because they are the future of bitcointalk.
Thank you!