From the spec sheet, under 7.3.2 Bus Timing/Electrical Characteristics, in table 7-7 DC Electrical Characteristics:
High-power Port 4.75V Min / 5.25V Max
Low-power Port 4.40V Min / 5.25V Max
So I'm going to guess the ones I tested that were low were designed for low-power devices. Didn't know there was a difference, so I guess I'll have to take a look at the labels vs output. I've always assumed that usb = 5v so it's good to know there's a wide range that the spec allows for.