As celenik puts it in Apo these are two separate pages from one book.
No two pages are going to be the same, therefore two different aspects of a Heroic Spirit are going to have different abilities.
There are times when the pages are similiar enough that the stats themselves are not different. Eg Young Lancer Li and Young Assassin Li.
Therefore E rank stats for Berserkers make senses when you consider that you're not summoning something that was given Mad Enhancement. You're summoning a page that has it.
There is no reason to imagine a Servant without the Mad Enhancement boost because the Mad Enhancement is what makes the Berserker a Berserker.
That is, it doesn't matter what is one rank under E rank just like it doesn't matter what is one rank over A rank (it's not A+ nor EX) because the Berserker in Berserker form has E rank.