1. I downloaded a fresh new Slack12-DVD iso image file. After checking its md5 and gpg correctness I burned it into dvd disc using k3b (without verifying).

2. I checked the integrity of the resulting disc by issuing :

md5sum /dev/hdc

after several minutes I got the md5sum that was different from the one I used at the point 1.

Maybe I am missing sth but these sums should be the same, isn't it ?

More, I checked the disc on itself using:

md5sum -c CHECKSUMS.md5 | grep FAIL

and the result was 100% OK. (No FAILs)

I repeated all of the above using another laptop and second disc with exactly the same effects. All md5 sums were the same and different from what I expected.

Is it normal ?

I might be wrong here, but I don't see why the md5sum of an iso file should be the same as the filesystem it contains.
That's the best possible result.

What's the problem?

I wanted to calculate the md5 sum from my freshly baken dvd disc that matches the md5 sum I used to check iso file on my hdd.

I was thinking that:

md5sum /dev/hdc

would give me the same md5sum. It didn't.

So, how can I properly calculate the md5 sum of the dvd disc in order to check it against reference md5 sum (not using k3b) ?

Is the output you got from this:
not good enough?

The sole purpose of providing md5sums for iso files is to give you something to check your download against. If your downloaded file matches, then you can confidently burn the image to DVD.