On Intel Core i7 “Haswell” processors, the CPU attains significantly better results in the GPGPU benchmarks than the Intel HD Graphics 4600 “GT2” GPU. How is that possible?

AIDA64 CPU benchmarks are heavily optimized for Haswell – and all other modern CPU architectures –, and they utilize all available instruction set extensions, such as SSE, AVX, AVX2, FMA or XOP as well as full vectorization.

Using FMA and AVX2, a quad-core Haswell's x86/x64 part can indeed perform exceptionally, way better than the GT2 iGPU. It is, however, much easier to write such optimized code for the iGPU in OpenCL than for the CPU using a machine code generator or x86/x64 assembly.