The overarching difference here is that the first version is branchless.
16 isn’t the length of any string here (the longer one, with NUL, is only 15 bytes long); it’s an offset into the return object (whose address is passed in RDI to support RVO), used to indicate that the small-string optimization is in use (note the lack of allocation). The lengths are 5 or 5+1+8 stored in R14, which is stored in the std::string as well as passed to memcpy (along with a pointer chosen by CMOVNE) to load the actual string bytes.
The other version has an obvious branch (although part of the std::string construction has been hoisted above it) and actually does have 5 and 14 explicitly, but is obfuscated by the fact that the string bytes have been included as immediate values (expressed as integers) of various sizes.
As for why these three equivalent functions produce two different versions of the generated code, all I can offer is that optimizers are iterative and heuristic algorithms; they don’t reliably find the same “best” assembly independently of their starting point.