Reason of super-linear speedup?

Hello and thanks again for this very nice tutorial,

https://github.com/kayceesrk/ocaml5-tutorial/blob/1179f3e3f932d79b3e87cdd4c870c89c1b8b6dc2/README.md?plain=1#L470-L471

Would it be possible to know (and maybe put in the tutorial) the answer to this question? I personally can't find it.