There is a lot more to it than simply ful burn and capacity. I think the accountants would love to have a mix of 777 deployed across the network......but BA don't have enough of them for that. Some 777 don't have first class, some don't have crew rest facilities etc etc. all of that that has to be factored in as well as seating capacity, cargo capacity etc.
Then they have to decide on how rotations integrate with each other eg. If an a/c flies to south Africa it will usually have to sit on the ground all day before flying a night sector back to base, something that is not desirable for the 777 fleet as much as the 744 fleet due leasing and ownership etc.
I suspect in the case of the LAX, the earlier one is usually the quieter of the three and they had to find some more 744 availability to operate othe routes and this obviously works out.