If you only want to send the full order data once per order, use your Common Notification URL setting (in Seller Admin > Account Preferences) rather than any product-specific Payment Variable Information URLs (when you enable "Send transaction data to a URL" in the product's settings).
The Common Notification URL setting was added to our system relatively recently; previously, the product-specific setting was the only way to pass order data to an outside URL. Our Integration submissions were intended to mimic the format and dataset of a PayPal IPN as closely and completely as possible, so we include all the order data by default, which developers can choose to parse down if they don't need to keep or use everything. "Too much is always better than not enough."
The 'item_cart_position' variable indicates which item in the order triggered a product-specific data submission, so e.g. if the second item the buyer added to their cart happened to have "Send transaction data to a URL" enabled, then our submission for that item would include item_cart_position=2, so you can tell that corresponds to the values for 'item_name2', 'item_number2', etc. pertaining to that particular product.
The current behavior is not likely to change in the foreseeable future.
