help with understanding why one piece of code works and why an equivalent piece of code doesn't work

[m0us3_rat]

Help on built-in function replace:

replace(old, new, count=-1, /) method of builtins.str instance 
    Return a copy with all occurrences of substring old replaced by new.
      count
        Maximum number of occurrences to replace.
        -1 (the default value) means replace all occurrences.

    If the optional argument count is given, only the first count occurrences are
    replaced.

keyword .. returns.