If a number is the first non-whitespace in the line, Fortran indentation assumes it is a line number and moves it to columns 0 through 4. (Columns always count from 0 in Emacs.)
Line numbers of four digits or less are normally indented one space.
fortran-line-number-indent controls this; it
specifies the maximum indentation a line number can have. The default
value of the variable is 1. Fortran mode tries to prevent line number
digits passing column 4, reducing the indentation below the specified
maximum if necessary. If
fortran-line-number-indent has the
value 5, line numbers are right-justified to end in column 4.
Simply inserting a line number is enough to indent it according to
these rules. As each digit is inserted, the indentation is recomputed.
To turn off this feature, set the variable