Question

Maimonides, in the laws of Sanhedrin, chapter 7, mishnah 3, writes: “As you already know, it is the witnesses who carry out the execution with their own hands, whatever the mode of death may be, because for them the matter is absolutely clear, since they saw it with their own senses. For us, however, it is only a report, since we have no knowledge except what we heard from them. Therefore God commanded that the witnesses themselves be appointed over the matter. And this is a wondrous thing.” I did not understand why Maimonides emphasizes here that this is wondrous, something he does not usually do. If the Rabbi has an idea for understanding this, I’d be glad to hear it.
Thank you