DATALENGTH is not like LEN !

Two main differences between DATALENGTH and LEN:
DATALENGTH returns the number of bytes used by an expression.

LEN returns the number of characters contained in an expression.

DATALENGTH supports expressions of any type.

LEN supports only string expressions.

declare @TestLength TABLE 
(
  char1 CHAR(10),
  nchar2 NCHAR(10),
  varchar3 VARCHAR(10),
  nvarchar4 NVARCHAR(10)
) 
INSERT INTO @TestLength VALUES('test', 'test', 'test', 'test')
INSERT INTO @TestLength VALUES('test   ', 'test   ', 'test   ', 'test   ')

SELECT
  LEN(char1) AS len1, DATALENGTH(char1) AS datalength1,
  LEN(nchar2) AS len2, DATALENGTH(nchar2) AS datalength2,
  LEN(varchar3) AS len3, DATALENGTH(varchar3) AS datalength3,
  LEN(nvarchar4) AS len4, DATALENGTH(nvarchar4) AS datalength4
FROM @TestLength