You're not missing anything. If the two where sourced from the same full node, they should show the same data. It appears that there are two distinct sources used for the graph.

By the way, the mempool chart of today indicates that the full node sourcing the mempool count chart was uncleanly shut down today, which is not reflected in the other count either. On a clean restart, it would have simply reloaded the persisted mempool state, but instead it took hours to build up transactions in the mempool. (Alternatively that full node predates Bitcoin Core 0.14.0 when the feature was introduced.)

You're not missing anything. If the two where sourced from the same full node, they should show the same data. It appears that there are two distinct sources used for the graphs.

By the way, the mempool chart indicates that the full node sourcing the chart was uncleanly shut down today, which is not reflected in the other unconfirmed transaction count. On a clean restart, the node should have simply reloaded the persisted mempool state, but instead the graph indicates that it took hours to build up transactions in the mempool. (Alternatively that full node predates Bitcoin Core 0.14.0 when the feature was introduced.)