I’m making the adult size of the Bulky Mosaic Beanie by MJ’s Off The Hook and I feel like I’m missing something with the gauge.  
I’m still kind of an intermittent beginner crocheter and I’m learning how to read patterns, so there may be something obvious I’m not understanding.  
The pattern says the slip stitch band gauge is 8 sts x 12 rows = 2 inches, but for the adult size it also says 60 rows should measure about 15 inches unstretched.  
If 12 rows = 2 inches, wouldn’t 60 rows be closer to 10 inches?  
The finished adult hat circumference is 18 inches, so the 15 inch unstretched band makes sense to me, but I’m confused about how that lines up with the gauge.  
I’m using Malabrigo Vientos, bulky weight, 100% superwash merino, 130 yd/100 g. I’ve tried a 6 mm, 6.5 mm, and now a 7 mm hook. With the 7 mm I’m getting about 12 rows = 2 inches, so I think I’m hitting the listed gauge, but I don’t see how 60 rows would get me to 15 inches.  
Am I reading something wrong or misunderstanding how the band gauge is supposed to work? I’m just worried about sizing up my hook too much and messing up the rest of the pattern.

Bulky Mosaic Beanie gauge help - MJ off the Hook pattern