Unbelievably, I actually have this pattern book and made that sweater for a niece of mine about 15 years ago!
You must be losing stitches during row 5 and not noticing in row 6 because row 6 is just purling.
So the 10-row pattern repeat is knit for rows, pattern (rows 5 and 6 three times) for six rows. Yes, 10 rows is one pattern repeat.
What size are you making?
I hate these kinds of instructions that are written with the different instructions for different sizes all in a row in parentheses!
My guess is that you are accidentally doing the wrong instructions for your intended size somewhere in row 5.
I see now looking at the pattern that I went through the whole thing with a highlighter to make myself read ONLY the size I was making all those years ago.
Try that and see if it works out better.
When you go to repeat row 5, you should indeed have the required number of stitches. They don't resolve themselves by row 10. You are just knitting row 5, purl a row, row 5, purl a row, row 5, purl a row. There isn't anyplace to gain stitches back.
Highlight your size and try it again. It's worth it; it's a very cute sweater.