When it is used, a natural gas-powered vehicle produces greenhouse gases. These are carbon chains. In addition to CO2, you have to look at the gas molecule itself. A gas molecule may escape along the way while the vehicle is being fuelled or repaired. A CO2 molecule leaving the vehicle's exhaust pipe counts as 1 molecule, but a CH3 molecule, natural gas, that escapes into the environment is the equivalent of 20 molecules burned. So the molecules lost throughout the circuit are punished 20 times more than 1 molecule leaving the vehicle's exhaust pipe.
That is what the studies often do not take into account. They take into account what comes out of the vehicle, but they do not take into account the entire chain that the gas must go through. It is not necessarily only the distance that the natural gas must travel, but also the way it is handled. Emptying a vehicle's tank by mistake has disastrous consequences, when you know that you have to multiply by 20 to obtain the CO2 equivalent.