I noticed Electrum has a hard time predicting the Target distance from the tip too (this started with the mempool spam).
I can't tell you why there's such a difference.
If you obtain number "278 MB" and "249 MB" from column "Memory usage", then you're comparing 2 different things. "Memory usage" refer to RAM usage to store deserialized unconfirmed TX while some website simply show total size of all unconfirmed transaction.
For other user, you can visit https://www.statoshi.info/d/000000020/memory-pool?orgId=1 to know the difference. Pay attention to chart "Mempool Transactions" and "Mempool Dynamic Memory Usage".