Question: Why does indets not list _Z

Just for my understanding

x^ln(x)-c;
solve(%,x,allsolutions);
indets((%));
print("but here it does:");
indets(%[-1])
                            ln(x)    
                           x      - c

                           /   ln(_Z)    \
                     RootOf\-_Z       + c/

          /     ln(_Z)        /   ln(_Z)    \        \ 
         { c, _Z      , RootOf\-_Z       + c/, ln(_Z) }
          \                                          / 

                      "but here it does:"

                          {_Z, ln(_Z)}
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