320GB Internal Harddrive Question

I wouldn't warry about it to much, but ill explain the most common reasons.

1. The major culprit is the fact that hard drive manufactures calculate the HDD space differently then the OS.

2. Some file systems require extra space for what ever reasons. In turn removing the space taken from the final free space calculation.

3. The Operating System needs some space for its kernal(core) files. Sometimes its factored in to the final space cost and other times it isn't.