What are possible causes of major differences in a number of transactions among mempools?

Question

There are only 2221 transaction in a mempool of a local, fully-synchronised Bitcoin Core 24.0.1 node running uninterrupted for 2 hours. In contrast, mempool.space reports twice the amount of transactions (approx. 5500 ). What may cause the difference? Is such significant difference among nodes' mempools expected?

Local node (default settings if not stated otherwise):

getmempoolinfo
{
  "loaded": true,
  "size": 2221,
  "bytes": 3756490,
  "usage": 16992656,
  "total_fee": 0.09189350,
  "maxmempool": 600000000,
  "mempoolminfee": 0.00000000,
  "minrelaytxfee": 0.00000000,
  "incrementalrelayfee": 0.00001000,
  "unbroadcastcount": 0,
  "fullrbf": false
}

Answer 1

Bitcoin nodes only relay transactions to their peers once when they first learn about them. When a node comes online after a transaction made the round, they will not learn about that transaction until it is either included in a block or some node (usually the sending wallet) rebroadcasts it. So, if your node has only been online for two hours, it will generally only know about the transactions submitted to the network in the past two hours.