1- initally:

stock 0 // price per stocked unit 0 // total stock price 0

2- recieve 8 products at cost 10

stock 8 // price per stocked unit 10 // total stock price 80 because 8*10=80

3- recieve 4 products at cost 16

stock 12(8+4) // price per stocked unit 12 (144/12) // total stock price 144 because 80+(4*16)

4- deliver 10 products at cost 12

stock 2 (12-10) // price per stocked unit 12 // total stock price 24 because 12*2

5- recieve 2 products at cost 6

stock 4 // price per stocked unit 9 (36/4) // total stock price 36 because 24+(6*2)

You no longer have those units on hand at the end, as some of them already got out at a price of 12, so it is not like a historical mean where you have into account all previous prices and each quantity you bought for each price. It is rather you update the value of the stock, wich means (as an extreme example):

If I buy 2 units at 10 each, and then sell them (so now stock is 0), and then I buy 2 more units at price 11, then what do you think my inventory value would be? According to your reasoning your value should be 10.5 (10*2+11*2)/(2+2), but that is not the case, the inventory valuation for each product will be 11€, as thats what they costed.

Hope I made that clearer.