That is correct for the bitcoin network difficulty.
But each pool has its own smaller difficulty for each miner because it is neither possible nor necessary for the pool to check every single hash.
For solo mining the Apollo runs a "mini pool" in the background.
... each pool has its own smaller difficulty... - This one has just blown my mind. What does that even mean?
To my understanding the pool would check a single hash ONLY if it is submitted as a valid block. There is simply no other reasons.
I know that Apollo runs its own pool, but I don't understand how could that pool have its own
smaller difficulty that is different from the bitcoin network difficulty? It makes no sense.
The network difficulty increases as more/faster miners find valid hashes.
If by "the network difficulty" you mean the bitcoin network difficulty, then of course I understand this as it is a core principle of the bitcoin network, but I cannot correlate it with the "Best", "Accepted" and "Rejected" Shares on the Apollo's Dashboard.
yeah that would work but with poor user experience.
You wouldn't get any stats until you submit a valid block share. For a solo pool like ckpool there is simply no other reason.
The pools difficulty is just a handshake in the stratum api.
For example:
you can send 1 share >100k with fluctuating stats.
you can send 10 shares >10k with smooth stats and balanced bandwith/ressource usage.
you can send 100 shares >1k with smooth stats and higher load
and without this limit you would (D)DoS the pool

As the others have said your "Best" should be > network difficulty for a valid block share.
Otherwise you can check that everything is working as expected (The Apollo generates a handful of accepted shares every minute).
Or just for fun to see how close your best share was but without any future correlation.
And you can see if your best share is statistically average or good/bad luck.