Smarty false is true?

Smarty false is true?

I have a function for smarty:
function smarty_function_false_() {
return false;
}
then if I do:
{if false_} false YES?? {else} false no {/if} <br>
{if not false_} not false yes {else} not false NO??? {/if}
And it evals to:
false YES??
not false NO???
I'm new to smarty, why is that happening?