isomorphism vs graph-subgraph isomorphism headers

[samsonsite1]

The docs mentions include files for two VF2 classes for isomorphism and graph-subgraph isomorphism: vf2_state.h vf2_sub_state.h

The docs specifically state:

"If the matching relation is graph-subgraph isomorphism or monomorphism, the role of the two graph is not symmetric. In this case, the first constructor parameter must be the smallest of the two graphs."

However, unless my eyes deceive me, the code in these files are exactly the same. There is no coding differences between the two. So, what's going on here?

Why are the docs telling me to use a specific header for a specific purpose, when the code is the same?