I think I've worked it out now

Just before making my note a couple of hours ago, I made my mempool more permissive via bitcoin.conf, and therefore I have lots of low-fee transactions in the (predicted) upcoming blocks:

The mempool.space that's running on my node marks them red if the fee rate is less than 2 sat/vB:

By contrast, the public one at https://mempool.space has a different colouring scheme, and therefore the same transactions that would be red in mine are green in the public one:

So basically, I think my mempool is relatively similar to the public one. For the next few predicted blocks, the number of transactions and the distribution of fees is relatively similar. So I think the main difference is this superficial different in the colouring scheme

( nostr:npub1uy6fplfrgddrex6ndcvwsvy7nlv4gadlw2j80vgkduk5ceg69e3sgzn9k0 )