If a pipe runs half to two thirds full, then you run 6ft on a 1.5 inch pipe, you have cut the air space above the waterline off from the atmospheric vent opening. That's why the trap arm distances are shorter in the upc. It's generally just more stringent engineering in the upc. In real life the trap might only siphon under very particular conditions. Such as you fill a sink and pull the plug. But most times having your vent within 6' of a 1. 5 trap won't siphon the trap. Most times it won't siphon even further. Where the line is drawn is just up to a vote from IAPMO which is made up of plumbers and engineers. People make presentations and have arguments at IAPMO meetings discussing things like pvc vtr pennatrations and how they need to be painted due to sun exposure, then someone says well what about the inside of the pipe it will still be exposed to the sun, then someone says it should be painted inside and out. Then someone says well won't that make the pipe smaller? I'm sure it's a great time, lol.
I believe IAPMO set out to make a code that could be follwed in any circumstance and be bullet proof in its design while being influenced by solely American design standards. If you look at some of the allowable designs in the ipc the engineering is very permissive and likley wouldn't stand up to every condition. Let alone that if a group of American engineers and plumbers wanted something to change in the ipc I doubt they would have very much influence.