MATime Series/Moving averages

in my study text, a solved example uses 3 years of quarterly sales figures to calculate the moving avg. thus, the example shows: the moving average (ma) as (q1+q2+q3+q4)/4 and so on. i understand this. ------------------------------------------------ in the next example, it gives MONTHLY sales for 2 years (x1 and x2). and it says to calculate "a moving annual total" following the previous example, the first moving total shd be (j+f+m+a+m+j+j+a+s+o+n+d)/12 BUT the first total is calculated as (f+m+a+m+j+j+a+s+o+n+d+j)/12, that is, it starts from feb of year x1 and ends at jan of x2... and so on.... why?