The server rules for converting a number to a string appear to be different than the rules used by the ISQL client to display a number.

select (1-0.4), STRING ( (1-0.4) ), STRING ( 0.6 ), 'blabla' || (1-0.4)
 1-.4 STRING((1-.4)) STRING(.6) 'blabla' || (1-.4) 
----- -------------- ---------- ------------------ 
  0.6 .6             .6         blabla.6           

The first column 0.6 is not "a number converted to a string", it is "a number displayed by the ISQL client utility"

The second and third columns shows numbers that were converted to strings by the SQL Anywhere server.

If you want the leading zero to be included in the resulting string, you have to do that in your code... in the SELECT statement, or in your client application.

For example...

SELECT .6 AS number1,
       IF ABS ( number1 ) >= 1
          THEN STRING ( number1 ) 
          ELSE STRING ( 
             IF number1 < 0 THEN '-0' ELSE '0' ENDIF,
             STRING ( ABS ( number1 ) ) )
       ENDIF AS string1;

SELECT -.6 AS number1,
       IF ABS ( number1 ) >= 1
          THEN STRING ( number1 ) 
          ELSE STRING ( 
             IF number1 < 0 THEN '-0' ELSE '0' ENDIF,
             STRING ( ABS ( number1 ) ) )
       ENDIF AS string1;

number1 string1 
------- ------- 
    0.6 0.6     
(1 rows)


number1 string1 
------- ------- 
   -0.6 -0.6