Zend_Filter_Input::_validateRule() has the following code:

/**
 * Evaluate the inputs against the validator chain.
 */
if (count((array) $validatorRule[self::FIELDS]) > 1) {
    ....
} else {
    $failed = false;
    foreach ($data as $fieldKey => $field) {
        ....
    }
    ....
}

But $data cardinality is always less than (array) $validatorRule[self::FIELDS] cardinality since $data elements are collected by iterating through it.

It's not a bug, but it reduces code readability ($data is always one element array).