Shock Wave, Chatot, and Super Effective Damage

[Shrine Sprite]

Today I decided that I wasn't going to be lazy anymore and start actually trying to complete my pokedex. And I decided to start with training my Kirlia. (I was planning on training one for online battling anyway!) Anyway, I managed to raise it to 28 when I decided to begin training it at route 119 (I think... it's the route just west of Sunnyshore) in hopes of Defeating Floatzel and Gastrodon with my new move Magical Leaf, and I end up running into Chatot. Of course I was planning to run away, but my Kirlia did know Shock Wave, a move that was Super Effective against Chatot, so I chose to battle it. The battle took a long while... as soon as I got it near fainting, it would Roost off the damage. But then I noticed something, my Shock Wave wasn't inflicting the same amount of damage each turn. It was as though the damage being done was bouncing up and down. One turn I'd take about half of it's HP, the next I'd take only a quarter. Then I noticed something else... it would only say 'Super Effective' every other turn, thus explaining why Shock Wave inflicted a strange amount of damage.

I still can't understand what caused this... Chatot is a Normal/Flying type, and Shock Wave is an Electric attack, so it should be super effective... Has anyone else seen this / knows what causes this?

Wild Chatot: Lvl 41, Male
Mimic (Calm Mind)
Uproar
Featherdance
Roost

Kirlia: Lvl 28, Male
Synchonize
Timid Nature
Shock Wave
Magical Leaf
Calm Mind
Destiny Bond

I just don't get it...

[Raikou Trainer]

Roost removes the Flying-type characteristic during the turn it's used. During that time, Chatot would be considered a Normal-type, not a Normal/Flying type, therefore Shock Wave would inflict normal damage.

[WhoKnows_WhoCares]

Yeah, i was going to say that...

It also makes Ground type moves hit flying types since the flying type is removed...

And Roost is now the reason you will probably never see a Pure Flying type pokemon, as you can't have a pokemon with no type, even for 1 move

[shinycubone]

I didn't know that! That's why Pidgey became NORMAL/FLYING then. Darn it, it's the whole reason my can't hurt birds anymore!

[Kikyo]

Wow. That's really interesting.
Well, I suppose that solves both shrine's mystery and the "no pure flying" mystery at the same time!