Reflects continued paucity of premium travel demand. 772 has only seven more J seats than sum of J and F on 321T, but 164 more Y seats. Two 772s have as much Y capacity as 6.5 321Ts, but only 2.5 321Ts at the front.
At the margin self-paying pax will appreciate the ability to get the 772 premium economy seats for the price of a MCE upcharge (or free if elite).