What you'll find a lot of the time in offices is that it's not necessarily about skill, it can be more about trust, relationships and favoritism unfortunately. One thing to consider is if that other assistant has been there for a while. If he or she has, then consider the position that the dentist is in - you've got one long term assistant who the dentist knows how they function, even with mistakes, etc, and the dentist knows that the assistant will still be there when you leave for dental school. What would the working relationship be if the specialist is seen as "snubbing" the older assistant in favor of the "young upstart" (from the prospective of the older assistant) who might only be around a few more months? Most dentists don't like change once things have established into a routine, especially if that change is going to cause drama later - even if it will improve efficiency - yes I know this defies logic.
Don't take this personally, and don't let yourself become jealous - this will poison your working experience and relationship. What you need to continue doing is take the job you're given and excel at it, that is the most important way to break the cycle listed above. Some offices it simply won't happen, and it will just continue on seniority or who is best liked, and there's not much you can do about it.