In the AR15 gas system, there are three big factors that affect the operation:
1. The diameter of the gas port
2. The amount of barrel before the gas port
3. The amount of barrel after the gas port (dwell time)
When you move the gas port back, you are tapping the gas where it is higher pressure. For example, the port pressure with a rifle length gas system is between 12,000-15,000psi. With a carbine length gas system, the port pressure is about 24,000-30,000psi.
The shorter travel time (for the gas to reach the port, go through the gas tube and begin unlocking the bolt) also means that the unlocking cycle begins faster, contributes to a higher cyclic rate and generally narrows the window where the rifle will function reliably.
The second issue is dwell time. When you have more barrel past the gas port than a 20" AR, the gas pressure flows through the gas port for a longer period (because it takes a little longer for the bullet to exit and the pressure to drop off). The result is a rifle that is overgassed for the design (i.e. a 16" barrel with carbine gas); but you can also have too little past the muzzle.
However, if you have too little barrel after the gas port, the gas pressure drops off before you can get enough gas to cycle the bolt. 10.5" ARs are hard to get running reliably because you have the combination of the shorter gas system AND a shorter dwell time to deal with.
All of the numbers in an AR (cyclic rate, bolt load, etc.) are designed around the idea that you have a 20" barrel with rifle length gas system. The further you get away from that, the narrower the window gets for reliability.
An 18" with a midlength gas system is going to be taking in gas at a higher pressure (shorter than rifle gas system) and it is going to have a slightly longer dwell time than a 20" rifle. The 18" with the rifle length gas system has the original design's gas system; but a little shorter dwell time that you can compensate for by opening up the port a bit.