Why is this?

This behaviour is not new to just Traffic X.  It used to happen with Traffic 2005 as well. And, if I recall, it even happens with the default AI from FSX (ie, without an AI program installed).  Although I may be wrong, that would seem to indicate the parking positions for fuel stations are the issue here, even with FSX defaults right out of the box.

I do know what you mean. Stuff like this can be confusing and seem quite daunting. With regard to AFCAD files, yes, you would see many references to "AFCADs" on various forums and download web pages. Basically, if people have used an airport editor tool (an AFCAD program) and edited an airport to add new stuff, or to move things around, or fix bugs, then they may wish to share their work with others. They upload the AFCAD to the internet so that others can download and "install" it in to Flight Simulator. It is a simple matter of downloading the AFCAD file and copying it in to the appropriate Flight Simulator folder on your computer. The next time you run FSX, the changes are there. Done. That's it. You copy the file, the changes are immediate. There's no special requirements, or anything technical to understand or do. One thing to note though is that it is obviously a good idea to remove the existing airport file otherwise you will have two files for the one airport and that can lead to problems (move it, don't delete it; always keep the original file as a backup). Usually the author of the AFCAD file provides some instructions about where to copy it and what to do with the existing file that's already there in FSX.