The brain has other differences that are probably a major influences on it's efficiency;

1) It's incredibly parallel, few operations happen linearly. Even modern GPUs don't really match that.

2) It doesn't run everything all the time. Parts of the brain not in use are shut down. Modern computers do this to some extend but CPU cores don't switch off parts of the ALU not in use. The brain has much better energy saving options, only 10-30% of the brain are active at a given time.