Each node maintains its own mempool. The mempool is a collection of unconfirmed transactions stored in a Bitcoin node's memory, waiting to be included in a block. However, the mempool is not uniform across all nodes due to the following reasons:

1. **Transaction Propagation Time**: It takes time for a new transaction to propagate across the network. Different nodes may hear about transactions at slightly different times.

2. **Node Policies**: Not all nodes have the same transaction acceptance policies. Some nodes might refuse to accept a transaction if its fee is too low, or for other policy reasons.

3. **Mempool Size and Limits**: Nodes might have different hardware capacities. Some nodes set limits on the maximum size or number of transactions in their mempool to prevent running out of memory. If the mempool reaches its limit, the node might start dropping low-fee transactions.

4. **Node Restart and Uptime**: If a node goes offline or is restarted, its mempool might be cleared, and it would then start building it up again based on new transactions and what it hears from peers.

5. **Mempool Eviction**: If a transaction remains unconfirmed for a long time (perhaps due to a low fee), some nodes might eventually drop it from their mempool.

Due to these factors, while there's a significant overlap, the exact contents of the mempool can vary from one node to another.

RBF or wait it out 🤷‍♂️