Just exactly my thought at some point while drafting the article...
just evolutionary randomness.
You are right with this.
Trees do not produce exactly RuBisCo but its variant form RuBisCO II.
Further digging reveals that RuBisCO is actually found in all photosynthetic organisms, including grasses, but it is not found in trees. The reason for this difference is thought to be related to the different evolutionary pressures and environmental conditions that have shaped the photosynthetic machinery of different groups of plants.
Trees, since being perennials, have evolved different strategies for photosynthesis and energy capture that are better suited to their particular habitats and growth patterns.
So I guess that's why they do not have the exact type found in grasses. Evolution has a strong link in it.