I saw some copyright notices containing 2 years in them like this Copyright (c) 2003-2005 Someone. All rights reserved. Why is that? I think I knew this once but I completly forgot...

Explicit copyrights are not necessary, nor is any form being enforced by authorities. Etiquette and usage before the Berne convention ask that the format be "© 2003 Someone. All rights reserved.", where the year is the date of first publication, but people increasingly append the year of last change to that notice, when it is different to the date of first publication.

Further reading.

The multiple year thing happens a lot with web sites where content is published at different times.

Or on games where the development time (and the pieces of technology and IP that goes into it) happens over several years.

Got it, thanks.