Whether you are a winning player or a losing player doesn't depend on how many starting hands you play. It's your total strategy and style, combined with your hand reading skills. I.e. holdem has three streets after you decide to play your starting hands. It also depends on how you play those starting hands.

So when you started playing less hands did you change the amount of hands you raise and reraise accordingly. Also did you change your post flop aggression accordingly.

And as Nova said, it could just be variance.