I think that was the point... it was designed to be Motormaster for the Combiner theme, and someone at Hasbro thought, what if we do the reverse of turning Legends Optimus into Motormaster and see if this Motormaster can be done as Optimus.
When that happened, they probably thought Optimus would have a better chance of selling if it was released first so that it didn't look like the recolour, or they just wanted the bigger name released first.
For me though, the truck mode doesn't fit either character. When I transformed Optimus into a truck, the red section looks like it has two fronts to it. Both front and back are rounded and have panels that look like windshields.