PyTorch Geometric directed graph shows wrong number of edges when converted to NetworkX undirected format

Question

I loaded the PyTorch Geometric dataset OGB_MAG, converted it into a homogeneous dataset, and then checked its number of nodes as follows:

import torch
import numpy as np
from torch_geometric.datasets import OGB_MAG
import os.path as osp
import networkx as nx
from torch_geometric.utils import to_networkx, to_undirected

path = osp.join('..', 'data', 'OGB_MAG')
dataset = OGB_MAG(path)
data = dataset[0]
homogeneous_data = data.to_homogeneous()
print(homogeneous_data)
--> Data(node_type=[1939743], edge_index=[2, 21111007], edge_type=[21111007])
print(homogeneous_data.has_isolated_nodes())
--> True

Then I converted it to NetworkX format and checked its number of edges:

nx_data = to_networkx(homogeneous_data)
print(nx_data.number_of_edges())
--> 21111007
nx_data = nx_data.to_undirected(reciprocal=False)
print(nx_data.number_of_nodes(), nx_data.number_of_edges())
--> (1939743, 21091072)
print(len(list(nx.isolates(nx_data))))
--> 0

The number of edges is different (21111007 with PyG vs 21091072 with NetworkX after converting to undirected). The number of nodes is the same though. We also see that PyG says there are isolated nodes, but NetworkX says there are none.

Any insight as to why I'm seeing this discrepancy?