The only thing I see is a small sticker with those specs on it and Folgertech....
I'm pretty sure no one at Folgertech assembles stepper motors, so the only calculated guess left is that it is a no name (chinese?) make without any guarantee on how close they match those specs.
If you want steppers with real specs and tolerances you will need to go for branded ones and these are more expensive and perhaps it's overkill on quality for what we do with our printers.
A stepper isn't that high tech product. But like with all mechanical components it's all about tolerances especially air gap, rotor (finishing of the poles), ...
Branded steppers with a part number have at least the following specs published :
motor type (unipolar, bipolar, ...), step angle and tolerance, rated current and resistance per phase, number of phases, voltage range, holding torque and inductance.
Perhaps mailing Folgertech could bring some more light on this?