It was a good way to confuse me at least. 
Going slightly off topic here, but anyway…
Unless Scala adds more guidelines than Java itself does, I guess whether you explicitly specify serialVersionUID depends on your preference. Are you happy with the default scheme where they are generated automatically but break on any source code change? Would rather specify the ID explicitly but then also have to remember updating it whenever you make a change that breaks backwards compatibility? I think the answer to that can differ based on the circumstances.
But if the Scala does not guarantee keeping the serialVersionUID stable, then the “generated automatically” option sounds rather less appealing. 