Probably because they do not really encode how anything works and because, probably by necessity, the growth of organisms is a swarm intelligence that is quite self-healing.
In particular with coinjoined twins, it's quite remarkable how much the systems for body development still produce something that connects the inner workings, which was obviously not it's “purpose”,but the self-healing growth mechanisms that corrects for errors simply leads to that.
Consider the Hensel Twins who have two mouths but their digestive system at some point merges in a way that is capable of digesting. The “tubes” of their digestive tract actually merge at one point, but they have two stomachs.
In particular with coinjoined twins, it's quite remarkable how much the systems for body development still produce something that connects the inner workings, which was obviously not it's “purpose”,but the self-healing growth mechanisms that corrects for errors simply leads to that.
Consider the Hensel Twins who have two mouths but their digestive system at some point merges in a way that is capable of digesting. The “tubes” of their digestive tract actually merge at one point, but they have two stomachs.