Definitive Illegal Formation Rules

[Pivac Momčina]

So I've made a formation, had 20% possesion because it's illegal (didnt know there are illegal formations at all back then), checked the forum, fixed the prob, everything is okay now.

I actually won that game, but I was stunned by the possesion, so I set up a friendly match and had the same possesion (understandable 20-80) and lost to a guy, that I won 2 days before that without a problem.

But my question is, that formation I used, it's a well known formation in real-life football (I was told it's called the MW formation), why is it illegal in the game? I understand its because of only 2 players in the last line but still, it's a good formation that shouldn't be illegal.
Please notice that DML, DMC and DMR are playing defensively, too.