I think it is because you alternate the slip and purl and not slip the slips and purl the purls, You start each row with a Purl and end the rows with a slip. I felt the same way........once I realized my mistake I corrected it and finished according to pattern and only the very top is able to be stuffed. I can slip my finger (only 1 fingers width) into the head. Just make sure you follow the pattern and not make the same mistake I did. See, not only did I inadvertantly add another stitch but my slipped stitches, after a few rows, were so tight that I had to purl all the way across (mistake, I know) just to give the stitches some slack. I had already broken a stitch earlier in the body section and I didn't want to repeat.
I bought another skein of Chenille and I plan on tackling this project again. It is a cute bunny plus I don't give up that easily!
FYI: The reason my slipped stitches were too tight is because I was slipping the same slipped stitches over and over.