On most PoS networks, the protocol has a target issuance schedule — new tokens created and distributed to validators as a reward for securing the chain. The resulting APY for an individual staker depends heavily on the total percentage of supply currently staked: if fewer people stake, each staker's share of the fixed reward pool is larger, and vice versa. This is why staking yields tend to fall as a network's staking participation rate rises.
Some networks supplement base issuance with a share of transaction fees (Osmosis, dYdX Chain) or MEV (maximal extractable value) captured during block production (Ethereum, and Solana via Jito). These additional sources can make realized yield meaningfully higher than the advertised base issuance rate.
Validator commission is subtracted before rewards reach delegators, and some networks apply their own inflation-adjustment mechanisms. As a result, the 'real yield' after accounting for token inflation can be lower than the nominal APY suggests — a staker earning 8% nominal APY on a token inflating at 6% a year is only gaining roughly 2% in real terms relative to the total supply.