FSA: LIFO, FIFO: COGS convert, understand, reserve, current

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for inventory on B/S, LIFO reflect more historical price, FIFO more current price.
for COGS on I/S, LIFO reflect more current price, FIFO more historical price.

why when convert COGS(LIFO) to COGS(FIFO), it is COGS(LIFO)-change of LIFO reserve? I thought it sould add it on. Don't understand.