The top 3¢ stamp embossed cover was mailed Nov 2, 1936 (weak year cancel, but marking on the reverse confirms 1936) from Massachusetts to Jamaica short paid and was assessed postage due of double the deficiency, 20 centimes. The postage to Jamaica at the time was five cents, therefore the cover is underpaid by 2¢ and charged 4¢ postage due, or 20 centimes (1¢ = 5 centimes), double the deficiency. The Jamaica post office applied the 2p marking.
The bottom cover was mailed from New York Jan 19, 1938 with 3¢ postage affixed to Maryland and redirected (forwarded) to Jamaica. Because the correct postage was initially applied the cover was assessed only the shortage for the 5¢ rate to Jamaica, or 2¢ (10 centimes). The Jamaica post office also applied the 2p marking to this cover.
Why were both covers given the 2p marking if different postage due amounts were assessed?