Why some delivery methods are not available for some customers?

Question

I have 4 shipping methods.

The old customer gets only 2 methods. I've created the same customer with another email. It gets all 4 shipping methods.

I dived into code and DB table sales_flat_quote_shipping_rate.

Sorting by address_id we have: for old customer: 2 rate records; for the same new customer: 4 rate records.

Why does it happens? and where is rate record created?

UPDATE 1:

I doesn't depends on address. It depends on product. If product has filled weight then customer cannot see some shipping methods. Now the question is why?)

Answer 1

The reason was in proccessAdditionalValidation function in Mage_Shipping_Model_Carrier_Abstract class:

    /**
         * Processing additional validation to check is carrier applicable.
         *
         * @param Mage_Shipping_Model_Rate_Request $request
         * @return Mage_Shipping_Model_Carrier_Abstract|Mage_Shipping_Model_Rate_Result_Error|boolean
         */
        public function proccessAdditionalValidation(Mage_Shipping_Model_Rate_Request $request)
        {
            $maxAllowedWeight = (float) $this->getConfigData('max_package_weight');
            $error = null;
            $showMethod = $this->getConfigData('showmethod');
            foreach ($request->getAllItems() as $item) {
                if ($item->getProduct() && $item->getProduct()->getId()) {

// here I got true

                    if ($item->getProduct()->getWeight() > $maxAllowedWeight) {                         $error = Mage::getModel('shipping/rate_result_error');
                        $error->setCarrier($this->_code)
                            ->setCarrierTitle($this->getConfigData('title'));
                        $errorMsg = $this->getConfigData('specificerrmsg');
                        $error->setErrorMessage($errorMsg?$errorMsg:Mage::helper('shipping')->__('The shipping module is not available.'));
                        break;
                    }
                }
            }
            if (null !== $error && $showMethod) {
                return $error;
            } elseif (null !== $error) {
                return false;
            }
            return $this;
        }