求高手解读一下这两个代码

data revenue2;
input Day : $3.
Revenue : dollar6.;
Total + Revenue;
format Revenue Total dollar8.;
datalines;
Mon $1,000
Tue $1,500
Wed .
Thu $2,000
Fri $3,000
;
run;

data revenue1;
retain Total 0;
input Day : $3.
Revenue : dollar6.;
Total = Total + Revenue; /* Note: this does not work */
format Revenue Total dollar8.;
datalines;
Mon $1,000
Tue $1,500
Wed .
Thu $2,000
Fri $3,000
;
run;

两个跑出来的TOTAL值不一样