<?php
require_once 'Zend/Validate.php';
$float = 0.000000234;

var_dump(Zend_Validate::is($float,'Float'));

// false

<?php
require_once 'Zend/Validate.php';
$float = 1.0000000234;

var_dump(Zend_Validate::is($float,'Float'));

// true

<?php
require_once 'Zend/Validate.php';
$float = 0.000050004;

var_dump(Zend_Validate::is($float,'Float'));

//false

<?php
require_once 'Zend/Validate.php';
$float = 0.005000004;

var_dump(Zend_Validate::is($float,'Float'));

//true

In other words: 100% valid floats are not considered a float.