the valuation of inventory at periodic weighted avg. method

Boomerang Co had 200 units in inventory at 30 November 20X1 valued at $800. During December it made the following purchases and sales.
2/12 Purchased 1,000 @ $5 each
5/12 Sold 700 @ $7.50 each
12/12 Purchased 800 @ $6.20 each
15/12 Purchased 300 @ $6.60 each
21/12 Sold 400 @ $8.00 each
28/12 Sold 500 @ $8.20 each
Calculate the value of closing inventory at the end of December using the periodic weighted average.

the answer for the question is $582

the method if calculation was as follows:

Periodic weighted average
= cost opening inventory + total cost receipts / units opening inventory + total units receipts
= (200 x $4) + (1000 x $5 + 300 x $6.6 + 800 x $6.2) / 200 + (1000 + 800 + 300)
= 12740 / 2300
= $554 x 700
= $3878
Periodic difference
= $4460 – $3878
= $582

Could you please explain why we are subtracting $4460 by $3878 to get the value for closing inventory?