Squid Memory Usage

Hello,

I have 384 MB of RAM and I set, 'cache_mem' to 120 MB (is this right?),
since there's a note that says process can become twice or three times
bigger than the value for cache_mem. But looking at cache manager's
General Runtime Report:

Memory usage for squid via mallinfo():

 Total space in arena: 61882 KB
 Ordinary blocks: 61826 KB 156 blks
 Small blocks: 0 KB 0 blks
 Holding blocks: 3116 KB 4 blks
 Free Small blocks: 0 KB
 Free Ordinary blocks: 56 KB
 Total in use: 64942 KB 105%
 Total free: 56 KB 0%

Memory accounted for:
 Total accounted: 54054 KB

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Why is that the 'Total space in arena' is not nearly equal to cache_mem
(120 MB)?  Why is the value for 'Total space in arena', 'Total accounted'
changing?  Why is 'Total accounted' less than 'Total space in arena'?