This is because the footer is cached and the cache key does not include the number of items in the cart.
But I don’t recommend including the cart count in the key.
You have to options here
1. Disable cache for the footer: Override the Mage_Page_Block_Html_Footer and replace the _construct method with this:
protected function _construct()
2. Include the cart count in the footer cache key: (The one that I’m not recommending. But if you still want to do it.... )
Override the same class (Mage_Page_Block_Html_Footer) and make the method getCacheKeyInfo look like this:
$key = parent::getCacheKeyInfo();
$key = Mage::getSingleton(’checkout/cart’)->getSummaryQty();
I hope it helps