|Quoting Brendows (Reply 1):|
Another issue is that the 77L has the same rudder as the 77W but is quite a bit shorter
I believe that the tail arm is the reason, but it's not the yaw axis as suggested above. Instead, it's the pitch axis. It's a certification requirement that the wing must be able to produce enough lift for 3.75 g's ultimate load. At that load, the h. tail must be able to trim the wing lift at the forward CG position. This means the h. tail will have a download for this condition.
Since the 772LR has a shorter tail arm than the 773ER, the 772LR wing will be producing more lift than the 773ER wing at the same weight because the h. tail download will be higher. Therefore a 772LR wing at 775K MTOW would need to be stronger than a 773ER wing at the same weight.
I suspect that for simplicity of construction, the 772LR and 773ER wings are the same structurally. This means the 772LR MTOW will be less than the 773ER, hence we see the 772LR MTOW limited to 766K.
Just a guess, but it seems to fit the geometric dimensions of the two models.