When I learned to crochet, I was taught that a slip knot was added to the crochet hook as a crochet technique alone. After that, you made a specific number of chains. I have never counted the slip knot as a stitch. Some people have been taught to count the slip knot. This will be another reason why the foundation chain count may be off when working a project. It will depend on how the crocheter was taught and how the pattern author was taught. This is something to keep in mind as a crocheter and deciphering the many patterns you come across. Each author is going to have a different interpretation in how they guide you to make a project.