... Well, I didn't really paid attention on the fight so I might missed something, but I know it's real...
... The effect was, "When this card attacks, the defender gets -1 ATK/DEF/STA". And don't forget the stats of the Marksman itself are quite high to begin with...
... Funny thing is, I once fought against a Marksman Knight that HAS an effect, even though it should be a card without effect... (not to mention the effect is quite bad)
I think it was the character card or an spell's effect, I've seen stuff like that before.