Since this is the back and, like you say you want both sides of the back to be the same, I think there is one important word missing in that set of directions.
I believe it needs to read "Dec 1 st at each end of the next 5 rows," etc. You will want the decreases on each end all the time. Another clue is that with the number of decrease rows given you would lose only 10 sts if you did the decs at only one end. 103-10=93, and they tell you you will have 83 at this point. Whereas 103-20 (2 per row)=83.
A hint (you may have heard): it is better to put the decreases 1 or 2 stitches in from each end rather than right at the end.