$_ vs !$. Last argument of the preceding command and output redirection

The question is about special variables. Documentation says: > !!:$ > > designates the **last argument of the preceding command**. This may > be shortened to !$. ---- > ($_, an underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the **last argument to the previous command** after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. There must be some difference I cannot catch, because: $ echo "hello" > /tmp/a.txt $ echo "!$" echo "/tmp/a.txt" /tmp/a.txt $ echo "hello" > /tmp/a.txt $ echo $_ hello What is the difference?