Hi Mr Mguyen. Thats normally the case, that is, any software remembers the location of the file you are working on EVERYTIME, regardless of how many open files you have. If you open project A, it will always save on project A location, no matter if you previously worked on project B,C, etc.
In Archicad, If you worked on project A, and then work on project B from another folder, project B will save on Project A folder if not careful.
Also, if you open more than one Archicad session, the software remembers the last location of the last file saved regardless of the session, so if you work on project A and B at the same time and save on B, then A will also save on B folder.
That is as far as i can remember, since im now behaviorily conditioned to use "save as" and always select the correct folder, which is, at the very least, annoying.