Using incorrect names on census

[GAJM1]

Hi I have a couple who had a family out of wed lock between 1900 to 1920 in Massachusetts. Now in the 1910 Census they used the wrong first names.  The first names they used are not their middle names or anything like that. So they were making the first names up. Would their have being a reason a couple might what to hide on census especially if they were not married and having children?

Castlemcauliffe

[Genealiza]

Could they have been "nicknames" used by the family?

[anitah1972]

Are you certain you're not looking at siblings or a separate pair of people altogether?   

Also: I'm finding that with the standard naming systems (First born male named after father's father, etc) that there can end up being several people of the same name in the same generation in the same area - and so middle names and nicknames seem to be common in order to differentiate.