Of course, after some interaction between the people, seniors members are used to merit those ones whose posts they find to be qualified for the task. Of course, this is a matter of probability: if you have been in here for a long time, and you are active and aware of the forum issues and circumstances, that's more probable for you to write something of use to the community. This is fairly normal, then, that senior users are the more rewarded ones.
This is fairly normal because as you become more involve in the forum, you will know how the community thinks and you can stimulate a better discussion with them. You can contribute other perspectives than what is popular and give new insights for a certain topic. This will you a higher chance to earn a merit.
But it doesn't mean that there are personal preferences,
There are personal preferences. I saw posts getting merit because of personal preferences (posts that are sarcastic, all about jokes, and etc). However, I think any user has different definition of "meritable posts" and we cannot judge them on their preferences. If I have a merit, I have the freedom to give to to anyone I think I deserve and you may find my "meritable post" similar to what your opinion of a post that deserves a merit.
in fact, newbies should be more rewarded, for there are many people out there just looking for some newbies to reward. The problem is that there are few, actually, deserving it.
The action to increase merit sources is for the Newbies that deserve merits. More merit sources means more merit for circulation. I think we need to commend Theymos in increasing merit sources (some users were assigned as merit sources even though they did not apply as one). This just proves that Theymos is looking for more ways to eliminate the shortcomings of the current merit system.
These merit sources are also active in searching for newbies that deserve merits. I always see Jetcash make a post with a certain question then if a low-ranked member answered it coherently then he will reward a merit. I know another one, (Pharmacist if I am not mistaken?) that is searching other boards than Meta to give merits to newbies engaging in helpful discussions.