zlib offers multiple compression levels, and of course there are other deflate implementations as well. The Zip file might have been created with a different de

zlib offers multiple compression levels, and of course there are other deflate implementations as well. The Zip file might have been created with a different deflate implementation, or with different compression settings. Some PNG optimizers really take it to the extremes and brute-force the most optimal (or at least close to optiomal) deflate encoding, which is not something normal Zip tools usually do. PNG filters might also play a role in here, because the difference to the previous pixel row may be easier to encode than the ASCII text on its own (there are fewer and smaller delta values to encode).