Where does the recursive variable expansion in bash/shell numeric contexts come from?

Where does the recursive variable expansion in bash/shell numeric contexts come from?

bash shell posix ksh arithmetic-expressions

shell

Is $(( expr )) equivalent to $[ expr ] in bash? [duplicate]

bash shell arithmetic-expressions

shell

In bash how do I divide two variables and output the answer rounded upto 5 decimal digits? [duplicate]

bash shell arithmetic-expressions bc

shell