Fields are normally separated by whitespace sequences
(spaces, TABs, and newlines), not by single spaces. Two spaces in a row do not
delimit an empty field. The default value of the field separator
is a string containing a single space,
" ". If
interpreted this value in the usual way, each space character would separate
fields, so two spaces in a row would make an empty field between them.
The reason this does not happen is that a single space as the value of
FS is a special case—it is taken to specify the default manner
of delimiting fields.
FS is any other single character, such as
each occurrence of that character separates two fields. Two consecutive
occurrences delimit an empty field. If the character occurs at the
beginning or the end of the line, that too delimits an empty field. The
space character is the only single character that does not follow these