It's not confusing really. To achieve perfect combustion, an engine needs 14.7 parts of oxygen for every 1 part of fuel. If you have less oxygen, your car will run rich, if too much then it will be lean. The ECU does a balancing act, going back and forth between slightly rich and slightly lean all the time, transparent to the user. That's why sometimes you'll find that for one tank of gas you get more mileage, then suddenly the next you get a bit less, everything else being equal.
When you travel up to Genting, although the ambient temp is cooler the air is thinner, that means less oxygen particles in the atmosphere. This is why some cars struggle when climbing, and overheat due to prolonged lean burn. The added stress of using a low gear and high rpm exacerbates the problem. For those cars, the cool air in Genting isn't able to counter the added heat generated from the lean burn, so having an efficient and fully working cooling system is paramount for Genting runs. This is very apparent with carburetted cars with fixed air-fuel ratios, less so with EFI.
On ground level, if you drive at night, do you feel your car being slightly more responsive than driving in the mid day sun? That's because cooler air is more oxygen-rich, allowing more work to be done given the same volume of air entering the engine. When the ECU detects more oxygen particles in the air that's coming in, it allows more fuel to pass through to be burnt, resulting in more power.
There are no lousy cars. Only better cars.