Marg, are you looking at a photocopy of the original certificate, or at a transcript?
Because if it is a typed or hand copied transcript, the transcriber will have decided what to write, and if there was a blank under parents' names on the original entry because the bride/groom didn't bother to fill it in / or forgot to fill it in / filled in by someone else who didn't know / etc., the transcriber could have interpreted that as parents unknown. There is usually a pretty mundane explanation.

Sue