Don't get me wrong,but...what makes you think that this player is "suspicious" ?
Did you check his blessings? Or do you know what items he has? There are some items in game which prevents you from being stunned (they don't protect you 100% but they do a pretty neat job) as well as some trade fair blessings. Before jumping on assumptions,I'd highly suggest you to check those,for example,in Private Sale there is a bezel which makes you stun more frequently,or an
item which is charged with 3 evil eyes and gives you an awesome blessing of protect-self-from-a-lot-of-things-and-bring-doomsday-on-the-enemies.
And from my experience,I think it's safe to say that people with blesses,items etc. of stunning tend to stun more frequently with the super-blows,so yeah,it's pretty normal. If he did not have any blessings,then it's most likely his items,I'd say.
Cheers.