As mentioned in the previous comic's commentary, this mistake wasn't planned. I looked back at that earlier comic, saw 7.9%, and applied it to Sarah's height. Thing is, the reverse of 7.9% shorter is actually a little under 8.58% taller (100 / 92.1).
While the difference is noticeable in a side by side comparison, it's small enough that I could have gotten away with just editing the dialogue in the previous comic. I didn't do that, however, because I liked Grace making an honest mistake like that. She's practically a living calculator, but she can still mistakes, and I like that.
This comic wound up being WAY more copy/paste friendly than I imagined it would be. Not only do I like Grace's maintained pose, but it turns out that, even with her head turning, Sarah's hair made it so Grace's silhouette doesn't change at all between panels one and three. I suppose it makes sense, but at the time, I found it to be a shocking discovery.