There may also be the assumption that the graph is acyclic.

Some causal models, while not flat out falsified by the data, are rendered less probable by the fact the data happens to fit more precise (less connected) causal graphs. A fully connected graph is impossible to falsify, for instance (it can explain any data).

Among all graphs that explain the fictional data here, there is only one that has only two edges. That's the most probable one.