Most cob songs are in D for the most part; it's a lot easier to play a D minor run (all white keys, how convenient!) than with a C minor fingering. And for the drop C stuff, he'd have to be playing with a Bb tuning, which is a major pain in the ass, probably one of the harder keys to play in actually.
So trust me. I've been playing piano/keyboard for 15 years...not that I have much to show for it.
This is really confusing.
I'm watching Chaos Ridden Years now and Janne starts playing this one note at the start of the guitar/keyboard battle and then doing a trill on just that note. The KEY he's holding is a D key, in between the C and the E.
The ACTUAL pitch of it is a C (5th fret, G string on Guitar Pro) but if you tuned GP down a whole step, you'd need to move it up to the 7th fret to keep its pitch. It's the same note that Alexi would get if he played the G string, 7th fret.
You can also see the lowest D key on his board taped down.
This note is the same pitch as Alexi's open E, A & D strings together considering the tuning he's using (CGCFAD). Alexi is playing in the postions of Dmin on his guitar but because they're both tuned down a whole step, it sounds like to a piano or something.
All this indicates to me that to play a D key and get a C note, he's tuned his keyboard down a whole step, right?
Also, Alexi's playing on this particular battle fucking sucks.
Damn right man, rawrkin' tab as usual. 
I wanna hear that shizzle!!
