What explains the variation in performance between the benchmark's C version and its AVX2 version?

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The variation in performance between the benchmark's C version and its AVX2 version can be attributed to the use of Advanced Vector Extensions 2 (AVX2) in the AVX2 version. AVX2 is a set of instructions available on modern CPUs that allows for more efficient processing of data in parallel.

In the C version, the code is not optimized for these instructions, and therefore, the processor may not be able to use its full potential. However, in the AVX2 version, the code has been optimized specifically for these instructions, allowing the processor to use them to their full potential. This results in a significant improvement in performance between the two versions.

Additionally, the AVX2 version may also be able to take advantage of other CPU features such as multi-threading, improved caching, and reduced memory latency, depending on how the code has been optimized.