The mempool is not actually a thing that is the same for everyone. Every node has its own mempool and it can freely decide to add transactions to it or remove some. It is possible that two websites have different rules set up in their nodes on when to delete transactions that haven't been confirmed for a long time, for example when the fee is too low.
But drops like this are still strange. Maybe blockchain.info deleted a bunch of spam transactions at once? I don't know.
Thanks for sharing!