Some confusions on sdshdr5 & sdshdr8 on key/value actual memory usage and MEMORY USAGE command

I did some investigation on Redis source code 4.0 while I was doing my job. Something about sdshdr5 and sdshdr8 storage had raised my curiosity and something really confused me popped up. Here are the steps to reproduce the scenario:
1. Open redis cli
2. Type SET key value
3. So here from my perspective and observation, THE value has been stored with sdshdr8 and THE key has been stored with sdshdr5(through dbAdd->sdsdup)
5. I was guessing with the help of MEMORY USAGE, THE key should be analysed as sdshdr5, while THE key was passed via c->argv[2]->ptr, it will be first encapsulated with method createdEmbeddeStringObject which adopt sdshdr8 as the struct type. So it will be analysed as sdshdr8 rather than sdshdr5.
So, could anyone explain what is the actual reason for this and will this affect the accuracy of MEMORY USAGE command? Appreciated