Instead of blindly accepting that 18 rows makes 10 garter ridges (it doesn't), I have to count rows. And recount. And try to see if there's any way that the 10 ridges could be due to fractions/rounding. (It can't.)
I eventually came to two possibilities:
1. There's an error in the pattern. (If there is, it's consistent between the two sizes.)
2. Saartje uses a long-tail cast-on or something similar that created the first row of knitting in addition to the cast on, but doesn't count it as a row.
I think it's #2, but instead of just doing that (or blindly following the 10 garter ridges, which only produces a small difference), I have to e-mail Saartje to see what she intended.
I'm waiting to hear from her. (Hopefully she's not as lazy about answering her e-mail as I am!)