Thomas Graves answered the questions correctly–he wasn’t asked to list all his previous wives. The question was “were you previously married?”
He answered correctly. He just didn’t indicate that there were two previous marriages before his current wife and that the second previous marriage ended in divorce. He only mentioned the previous wife who had died in 1879.
The five-year gap does not mean that there had to be another wife. But the gap does suggest the researcher try and find out what happened in the interim. In this case the 1880 census enumeration may be helpful.
Any gap in time in any chronology is an opportunity–an opportunity to see what else can be located.
And remember that anyone can leave anything out of an application and it always pays to read the question exactly and precisely.