Rim offset book vs actual
 

Hey guys. In case anyone didn't know, on factory rims for F150's there is usually a stamp inside of the rim specifying the rim diameter, width and offset. I was visiting a few tire places and they all referred to "the book" for what the stock offset was and they all said 30-40mm. I took my rim off today and saw my rim was actually 44mm positive offset. Can someone explain to me why the book is so off to what the offset actually is? Wheel-size.com has confirmed the 44mm offset for 2014 F150 factory rim. One other thing is as you can see in the picture there is a J and an X in the rim description.
18X7.5JX44
What do the letters represent?

I think all wheels have the specs stamped on them....J refers to the shape of the wheel where the tire bead is, X means it's a one piece wheel

Thank you for that info Cunmud.