ISO: Pierogi question!

My maternal grandmother and grandfather grew up in Poland, so I grew up eating pierogis -- the potato kind, and ALWAYS with a spoonful of sour cream on top. There are a lot of polish-descended women in my office, and I found out recently that none of them have ever heard of putting sour cream on a piergo. I know this isn't uncommon, but what could account for it? Are there regions of Poland where sour cream is or isn't used? Or is it a local anomaly, or what? (A local fair has a polish food booth and they always have sour cream on hand. This didn't convince the women in my office. They look at me like my grandparents must have just pretended to be Polish.)