When issuing the df -h /dev/hda3
the size, amount used and space left do not match.

In my case df -h /dev/hda3 reports the following:
Size Used Avail Use%
164G 147G 8,4G 95%

This partition is not used for mounting the root file system. The partition system is ReiserFS.
What can I do to fix this. Or what is the cause of this.

Thanks in advance.

There is a logical explanation for that. I got it from my teacher a couple of weeks ago. In Windows, when you fill up the C: partition, your system starts acting slow and eventually crashes if not enough space is freed on drive C:.

Linux being the ultimate system solves this, by setting aside some space on every partition to resolve this issue. This way users can fill up the partition to it's maximum allowed, and the system would never get affected because of lack of space, simply, because there is no lack of space, as that space is reserved for the system.

Thanks for the reply,
I thought this was only the case for the partition where the root file system was mounted on.
Still, isn't 8,6 Gigs a little much?

I'm only asking because I have a dreambox (google for it if your interested), and I use it to record/stream tv shows to my linux server, so I don't have to use an internal hard disk.
Whenever I program the timer on it, it reports how much gigabyte's and time there's left on the server.
So it gives 17 gb and 8 hours left, now this is annoying when in reality it's only 4 hour's or something.