Motors are simple. My blind guess is that it's a mixture of improved insulation and improved heat flow.
Though speaking of heat flow, stand-alone fridges always come with requirements that they need a certain amount of space for the heat to escape the rear of the fridge. Builtin fridges don't have that heat. I now find myself wondering whether builtin fridges can ever be as efficient as standalone ones.
And you're right that it can be advantageous in the short term for quality companies to cut costs by cutting corners while coasting on their reputation. Who knows, AEG might be doing that right now. Your AEG beard trimmer makes me regret we didn't go with Bosch or Miele after all.
I also read about people plugging water circuit on top to tranfer heat into a hot water container, it's probably useful for a hot shower..
How these built in fridge done ? how many ? (I guess a standalone is made in more numbers so they have more know-how and mileage)
Also brands are deceitful after some time. At once they meant skills (say 60-90s SONY) but after a while, on mainstream product it's mostly a tag. For instance I found an AEG beard trimmer and was thrilled because AEG means gold in my eyes, when opened, it was a generic chinese PCB / assembly with a cute and sexy AEG case (you pay for the rubber).