bison precedence is useless? it doesn't work

I have declared such precedence for bison :

    %left '+' '-'
    %left '*' '/'

Recursive rules for arithmetic:

exp:       exp binary_op exp { .. }
           | literal_exp     { .. }
           | ID              { .. }

binary_op: '+'               { .. }
           | '-'             { .. }
           | '*'             { .. }
           | '/'             { .. }

I have an arithmetic expression: 10 * 3 + 5

My program calculates the sum, and it's 80! I still don't know why precedence doesn't work.