What is slippage?
Slippage is the difference between the price you expect and the price you actually get, caused by your own trade moving the market - it is worse when liquidity is thin.
On low-liquidity tokens, even a modest buy or sell can incur large slippage, so you receive far fewer tokens, or far less SOL, than quoted.
Deeper liquidity reduces slippage, which is one more reason PumpScore weights liquidity so heavily.