Note that higher levels are not guaranteed to be better

README.md

@@ -66,7 +66,12 @@ More advanced options can be found by running `oxipng --help`, or viewed [here](
 Some options have both short (`-a`) and long (`--alpha`) forms. Which form you use is just a
 matter of preference. Multiple short options can be combined together, e.g.:
 `-savvo6` is equivalent to to `--strip safe --alpha --verbose --verbose --opt 6`.
-Note that all options are case-sensitive.
+All options are case-sensitive.
+
+Note that oxipng is not a brute-force optimizer. This means that while higher optimization levels
+are almost always better (or equal) to lower levels, this is not guaranteed and it is possible in
+rare circumstances that a lower level may give a marginally smaller output. Similarly, using Zopfli
+compression (`-Z`) is not guaranteed to always be better than without.
 
 ## Git integration via [pre-commit]