That is because other expenses come (primarily) from purchases. Expenses in the Inventory - cost account are added only by sales transactions. The current asset value of the inventory item sold is transferred from Inventory on hand (or another custom control account, if you’ve assigned an item elsewhere) to Inventory - cost at the average cost. That average cost is not made up of a single, or even a few, purchases, but incorporates the result of your entire purchase history for that item. There could be hundreds of previous purchase invoices, debit notes, and even credit notes that affect average cost at any given moment. But in an average cost inventory valuation system, the average cost is maintained as a moving average, not as a list of specific transactions from which an average is computed. When there is a new acquisition of an inventory item, the cost of the new units are averaged with the existing units to calculate the new average cost.
For example, if you bought 10 units for £20, and 30 units for £30, Manager knows you have 40 units with a total cost of £50, giving you an average cost of £1.25 each for the 40 units. If you buy 10 more units for £25, the new average cost is [(40 x 1.25) + (25)] / (40 + 10) = £1.50. Notice, the calculation used only the prior quantity, prior average cost, and new cost and quantity to calculate the new average cost. The program has already disregarded the fact that the first two lots included quantities of 10 and 30, as well as what their individual lot prices were. This is the big advantage of perpetual moving average inventory valuation: you do not need to keep track of specific past transactions to properly determine cost of goods sold. (In other systems, such as FIFO and LIFO, you have to track those specific purchase lots. And that’s a big computational burden.) With the average cost method, the calculation never gets any more complex than the equation shown above, even after hundreds of purchases.
No, because cost of goods sold is fundamentally unlike any other expense category.
You don’t, because there are no purchase invoices directly associated with the Inventory - cost transactions.
If I understand what you are interested in, you are asking the wrong question. You seem to want to know which particular transactions contributed to the balance of the Inventory - cost account, which is another way of saying to the cost of goods sold. To know that, you would need to list every transaction that has contributed to the average cost of the inventory item at the moment it was sold. Under an average cost inventory valuation system, such a list is not maintained.