where is the memory ?

I have 512MB RAM installed with xp OS in my system. In window Task Manager, under performance Tab, The physical memory panel shows:
Total =457000;
Available = 88500;
Sys Cache =187000.
Now, where did I lose 55MB of memory? Again I assume that available memory value + system cache value must add up to Total value. But here it works out of just 275.5 MB. Where did the rest go ! Please enlighten this ignorant. Thanks.
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