My thoughts on what appear to be 2 virtually identical happening to different families.
All through the 19th century men left their wives and lived with other women and the wives never knew what had happened.
By 1921, 21 years into the 20th century and the men having fought in WW1 things were different.
I wonder if the men realized their wives may find it much easier to find them and chase them for money?
So they made decisions to deliberately not appear on the census thinking the chance of being found out and prosecuted for not filling in the census form was tiny compared to the chance of their wives finding them. Maybe wrong but you never know.