mem_cache + module, sometimes cache seems not to work

[Chuck Stein]

I have my own module that supplies tiles of data  I use mod_cache and 
mod_mem_cache to minimize the disk access of my module as there are many 
requests for the same tiles.  I am running on a LAN with a fixed number 
of clients and using mpm worker.
  StartServers                    2
  MaxClients                  192
  MinSpareThreads        25
  MaxSpareThreads        75
  ThreadsPerChild           64
  MaxRequestsPerChild   0
From my modules logs I see the second child-server created makes 
repeated accesses to to the same tiles.  This should not be happening, 
the repeated requests should not even get to my module because they 
should be served from cache.  The vast majority of tiles are served from 
cache.  I am seeing this when using ab (benchmark tool) with a single 
request so the tile is not getting pushed out of cache.  Out of 70,000 
requests for the same tile, 64 are getting through to my module when I 
would expect one or two.

Any ideas appreciated,

Chuck