Why does the Polygon PoS Chain Average Gas Limit (per block) keep changing?

Question

Most EVM chains have a fixed gas limit and gas target per block. For Ethereum, it's 30M and 15M: https://etherscan.io/chart/gaslimit

For Polygon, the gas limit seems to change every single block. How is that calculated, and where is that documented?

You can see that the gas limit is slightly below 30M as of Nov 2022 and gradually declining: https://polygonscan.com/chart/gaslimit