Tomcat6.exe process memory consumption

Hi all,
I am having an issue and I want to know reason for the same. I am currently having Apache Tomcat 6.0.32 as a windows service. My Memory configurations are as follows:

Initial Memory pool : 256 MB
Maximum Memory pool : 1024 MB
Thread stack size : 1024 KB.

Other VM memory params are
-XX:PermSize=64m
-XX:MaxPermSize=256m

I have deployed few applications on this setup and after a period of time, through JConsole I find the following memory observations:
Heap
====
Used: 361,371 kbytes
Committed: 488,956 kbytes
Max: 1,046,464 kbytes

Non-heap
======
Used: 181,080 kbytes
Committed: 273,984 kbytes
Max: 311,296 kbytes

If we add the "Committed" memory in both the cases, it comes to 488,956 + 273,984 = 762940 which is approximately 762MB.
But if I go to task manager and see the memory consumption of tomcat6.exe it show 1148644 which is approximately 1148 MB.
Now the issue is , why there is a difference of 386 MB additional consumption? This difference increases as the heap gets used more and more. Are there any other memory reservations specifically for tomcat6.exe since its run as a windows service? Please provide an answer if you know.