* in your commands is expanded by the shell before tar sees them. It also does not expand hidden files.

So when you do admin/* the shell expands to all non hidden files inside admin. Which does not include admin/.htaccess. So tar is never told to archive this file, only the other non hidden files and folders. It will still archive hidden files and folders nested deeper though.

In the second example * expands to admin and the other does which are not hidden at that level. Then tar can open these dirs and recursivly archive all files and folders including the hidden ones.

You can see what commands actually get executed after any shell expansions if you run set -x first. Then set +x to turn that off again.

Here is an example using ls:

$ set -x; ls -A foo/*; ls -A *; set +x
+ ls --color=tty -A foo/baz
foo/baz
+ ls --color=tty -A foo
.bar  baz
+ set +x