Version 1.2.11 has these key improvements over 1.2.10: That the similarity is completely coincidental.
If this page seems suspiciously similar to the Welcome to the zlib home page, web pages originally created by ( Not Related to the Linux zlibc Compressing File-I/O Library) (Also Free, Not to Mention Unencumbered by Patents) A Massively Spiffy Yet Delicately Unobtrusive Compression Library