Decoding only a certain frame results in different values than when decoding the entire file

Hello!

I am still using version 1.0.4.

When I have encoded a larger file and then only want to decode only one 
frame from it (for example the 2nd frame), is there anything special 
that I have to keep in mind?

I thought that I could simply read the bytes from the encoded file for 
example from the start position of the 2nd frame and then decode it. I 
did that, and I can play it play, but the values in the decoded 2nd 
frame are different than they are when I decode the entire file and then 
look at the values for the 2nd frame.
I hope I could explain it well.

I noticed that there is a pattern in the beginning of the encoded file, 
and I am not sure if I have to take it into account somehow.

It would be nice if someone could tell me if something comes to his/her 
mind that I might be doing wrong.

Thank you very much.

Hermann