Manager is using perpetual weighted average cost method. It means it re-calculates average cost per item after each purchase (or inventory write-on/write-off). Here is an example:
Jan. 1 Beg. inventory 5 units @ $500/unit
Jan. 5 Purchase 20 units @ $550/unit
Ave. cost = (2500 + 11000)/25 = $540
Jan. 9 Sales 21 units @ ave. cost of $540 = $11,340
Balance : 4 units @ $540 = 2160
Jan. 18 Purchase 6 units @ $600/unit
Jan. 25 Purchase 10 units @ $620/unit
Ave. cost = (2160 + 3600 + 6200)/20 = $598
Jan. 29 Sales 8 units @ ave. cost of $598 = $4784
Cost of goods sold on Jan. 29 = $4784