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I did ask the authors of zlib.  It seems that everything is fine 
except the assumption about "bigger is better" which seems 
natural at first ;-)  The monotonic test should add some 
comment that it may fail. See below:

On Wednesday, August 13, 2003, at 01:45  AM, Petr Prikryl 
wrote:
> My question is: Are the assumptions about the monotonic
> results correct?  In other words, is the Python test correct,
> or is the implementation of the Python wrapper correct
> (i.e. of the enclosed C code), or is there a bug in the
> implementation of deflate()?

[and Mark Adler answered]

It's a feature.  There is no problem with deflate or with the 
Python wrapper.  The compression results, especially on small 
files, are not necessarily monotonic with compression level.  
The compression level determines how hard deflate looks for 
longer matches.  However slightly longer matches do not 
always improve the compression, since it takes more bits to 
encode the longer distances back those matches occur.

mark