The floor is simply the cheapest listing right now. It is quoted as if it were a market price, but it reflects one seller's ask, not depth, and the best bid can sit far below it.
Manipulation is straightforward: withdraw the few cheapest listings and the floor jumps, with nothing traded. Conversely a single forced seller can reset the headline for the whole collection.
Example: a collection shows a 4.0 ETH floor, but the bid side holds only three offers at 3.1, 3.0 and 2.8 ETH. Selling ten items immediately averages well under 3 ETH. Judge NFT liquidity by the bid stack, never by the floor.
Related: nft, wash-trading-crypto, royalties-nft, market-depth-crypto