First of all, I like thick necks and it has been a real problem finding stock necks which accommodate me. While I used to adhere to the belief of measuring with calipers to give me an idea of what the neck would be like (buying online). I have come to appreciate the shape contributing more to the neck itself, that is, the volume of the neck. Currently, I am refinishing two necks, both measure .95 at the first fret and 1.0 on the 12th. However, one neck feels considerably thicker as it has a D shape. If I measure with a flexible tape measure around the neck, its 1/4 longer around. Therefore, when it comes to neck size, I am far more comfortable measuring around the neck, to get a better picture of the volume of the neck rather than the thickness. You wrap your hand around the neck so why wouldn't you consider measuring it that way? I think companies who advertise their neck girth are not being accountable, although not on purpose probably, of how the neck would feel.
Example, if you were do go on a blind date, would you like to know just the height of the person or their height and weight? To approximate volume of course.