Yes, it could very well be from another club. It is truly "career" stats which means every game he is played in regardless of any club filters that may be applied. His front page stats are those where it is filtered for your club only.

So if he took a wicket while playing for the opposition then it would count for the achievements.

You could work around it by creating another player record for him for his matches with other clubs and replace him with it for the matches where he played as opposition. Another option would just be to remove his name from the opposition bowling list and just leave it blank.

Or even better - give him a bowl and let him take a wicket for your club!