Winning contests I have never seen before

I wonder if anyone else has noticed this and if SH, @RachelSH can explain. I sometimes see winners in contests that I have never seen before. I look at them and wonder why I have no entries in that contest. It’s like I was excluded from seeing the contest from the start. But even when CHs choose a contest that only allows certain contestants, I am normally able to participate in those so I am wondering why I see winners in contests it seems I never knew about? Anyone else noticed this?