Why is disk usage reported by the OpsCenter storage capacity widget different than "Data Size" metric?

DataStax OpsCenter for Cassandra - why is disk usage reported by storage capacity widget different than "Data Size" metric? The Storage Capacity widget shows 38.96 TB used but a graph of the "Data Size" (data-load) metric (for the "cluster wide") shows 14.30 TB used. This cluster has two data centers and a RF of 3 for all tables. Here is an example image of the Storage Capacity widget