Also, I’m not sure if mempool accounts for it, however you can bump transaction fees using a child transaction.
Let’s say I pay 150sat/vB, realise i need it sooner, I bump it with a second transaction for 50sat/vB, being slightly imprecise, I’m not sure if mempool counts that as a 200sat/vB or 150 and 50 in their statistics. So many of the smaller transaction fees could be trying to bump (stuck) existing higher fee transactions too. You could write a script to investigate it.
Obviously two transactions means more data, so eventually they clear and get mined and the transaction count can drop, as people’s initial transaction fee is higher/better calculated based on their needs.