it's because of the amount of high fee transactions in the mempool. Currently, there is a thinner layer of high fee txns in the pool, so the average ends up getting pulled lower. At the end of 2017, there was an attack where they were flooding the mempool with extremely high fee transactions so every block was fully saturated with the highest fees. This is why the mempool chart is awesome but harder to read about what ultimately happened on chain. The mempool chart is a snapshot of unprocessed transactions whereas the Dune chart is what is actually in the blockchain.