When you list the directory contents with the
ls command, it will sort the listing into alphanumeric order according to current locale's sorting rules by default. It is easy to assume that this is the "natural order" of things within the filesystem - but this isn't true.
Most filesystems don't sort their directories in any way: when adding a new file to a directory, the new file basically gets the first free slot in the directory's metadata structure. Sorting is only done when displaying the directory listing to the user. If a single directory has hundreds of thousands or millions of files in it, this sorting can actually require non-trivial amounts of memory and processing power.
When the order in which the files are processed does not matter, the most efficient way is to just read the directory metadata in order and process the files in the order encountered without any explicit sorting. In most cases this would mean the files will be processed basically in the order they were added to the directory, interspersed with newer files in cases where an old file was deleted and a later-added file reclaimed its metadata slot.
Some filesystems might use tree structures or something else in their internal design that might enforce a particular order for their directory entries as a side effect. But such an ordering might be based on inode numbers of the files or some other filesystem-internal detail, and so would not be guaranteed to be useful for humans for any practical purpose.
As @A.B said in the question comments, a
find command or a
ls -f or
ls --sort=none would list the files without any explicit sorting, in whatever order the filesystem stores its directories.