This question was asked to our HOT years ago. The reply followed the lines of: you are asking the a/c to climb and accelerate. There is a ratio between the 2 parameters. TOGA gives 60/40 to climb/accel & LVLCH give 60/40 to accel/climb. True or not I have no idea. It could have been B.S. or true Boeing stuff. Either way, in his opinion, TOGA should remain until the takeoff is completed (it is a takeoff FMA) and then an en-route FMA should be displayed. Neater. You could stay TOGA & N1 to CRZ if you wanted, but it is untidy.