TOTAL=TOTAL+SCORE 之后，TOTAL的值被增加了SCORE的数值，如果TOTAL原值为0，则新值是SCORE。
TOTAL+SCORE之后，TOTAL值没有变化。

When there is a missing value in the score, the total = total + score produce a missing value.

The sum(var1, var2, var3...) ignore missing value.

The total + var is for vertical adding.

see the following example.

data test;
input a b c;
datalines;
1 2 3
2 . 4
4 5 6
7 8 9
;run;

data total;
   set test;
   if _n_ = 1 then do;
      total_ab = 0;
       total_a = 0;
          end;
   sum_abc = sum(a, b, c);
   total_ab = total_ab + a + b;
   total_a + a;
run;
proc print data=total; run;