Here is some information the MTG Flexible Thread Gauges that should help you help you understand the difference. There are links to the MTG type and the Snap Gauge style. The third link is to the MTG website.
There is wealth of information on the Ring-Plug-Thread Gage site.
Adjustable thread ring gauges can be calibrated to a Thread Set Master (or Thread Set Plug).
A fixed thread ring gauge cannot be calibrated and unless the Thread Set Master is purchased with the fixed Thread Ring Gauge there really is no way to determine if the fixed thread ring gauge his bell-mouthed or worn on any of the angles etc.
The cost of an adjustable Thread Ring Gauge is well worth it.