help with the instruction "at the same time"

i will go straight to the point i am a beginner knitter and i am struggling with the instruction “at the same time” of Kim Hargreaves pattern

it says,
20 sts
work 1 row
dec 1 st at armhole edge of next row, then on foll 4 alt rows, then on foll 4th row and AT THE SAME TIME dec 1st at neck edge of next and 2 foll 4th rows.
11 sts

i couldnt figure out why 11 sts i got less than 11. so if one can help me with this
thank you

One way to make these instructions clearer is to write out the armhole and neck decreases separately.
If you call the next row, row 1, then dec at the armhole edge on rows 1,3,5,7 and 9 and then on row 13. Dec at the neck edge on rows 1,5 and 9. That’ll total 9sts decreased.

okay now i understand …thank you so much i appreciate your help