When some plugin (maybe script too) redefine some WeeChat command than help should be shown for this plugin command not for WeeChat command.
Example:
Lets redefine 'topic' command like this:
cmdOpHandler = plugin->cmd_handler_add(
    plugin, "op",
    "gives channel operator status to nickname(s)", "", "", "",
    &cmdTopic, NULL, NULL );
Then there are no parameters for 'topic' command.
But when we do '/help topic' then WeeChat print help for internal command not for plugin command which will be executed.
[00:28:08] [i]  /topic  [channel] [topic]
[00:28:08]
[00:28:08] get/set channel topic
[00:28:08]
[00:28:08] channel: channel name
[00:28:08] topic: new topic for channel (if topic is "-delete" then topic is deleted)