Audiobooks play order ?

I put two MP3 files under the “Audiobooks” folder each day.  They are always named the same: RA.mp3 and RB.mp3.  One day it puts RA first in front of RB, the other day it puts RB first before RA.  What kind of sequences is it based on?  It does not seem to be based on alphebatical order of the file name.  These files are not tagged.

Finally, just create another playlist in the SD card, with these two files listed in order, then just play the playlist instead.

Really wondering what is the purpose of putting them as audiobooks, just treat them as another music is fine.