Gas fee lottery question

Anyone knows why this happens:

Sometimes I check on Ethereum Gas Tracker for a proper fee and let's say it's around 100ish GWEI, than 2 things couls happen,

– I set a transaction for 95 and sometimes it could take even hour (or days!) but it stays in the transaction pool and at least it is processed.

– I set a transaction for 95 and after a couple minutes transaction is straight away discarded or aborted , losing my fee.

Why sometimes miners keep transactions ethernally and sometimes it is discarded out of the blue? Who decides?

Thanks anyone for answering!