If you consider the documentation of the function you will see
def is_subgraph(self, other, induced=True):
"""
Tests whether ``self`` is a subgraph of ``other``.
.. WARNING::
Please note that this method does not check whether ``self``
contains a subgraph *isomorphic
Graph("E`oo") is the disjoint union of 2 triangles. Graph("ExSW") is two
triangles with 2 more edges connecting them. So the first graph is a
subgraph (a non-induced subgraph) of the second graph. But Sage reports
that it is not. Both graphs have order 6.
sage: h=Graph("E`oo")
sage: g=Graph("Ex
