Expired fingerprints

I got a notice from the INS (or BCIS as they call themselves now). Basically, I have to go into one of the INS offices to get my fingerprints taken AGAIN. According to the INS, my fingerprints have "expired". How the heck can fingerprints "expire"? Did I get new fingerprints? Am I totally in the dark about how, over time, your fingerprints somehow morph into something different?