help on a name query if possible

[claybury]

CAN ANYONE GIVE ME AN IDEA WHY ON TRACING A PAST FAMILY MEMBER ,THE CHRISTIAN NAME IS DOWN AS EVAN ON ONE CERTIFICATE AND ON THE CENSUS AS WILLIAM ? THIS PERSON WAS WELSH,1 AM WONDERING IF THERE REASON TO EXPLAIN THIS CHANGE OF NAME. MANY THANKS FOR ANY HELP.

[CheekyChops]

Are you 100% certain they are both the same person? Did he have a middle name? Is it just one census where he is referred to as William or more than one?