Can't comment on your family specifically, but one of my great Uncles was Christened and buried with just one Christian and one surname, as he was on all the census forms until his death in 1945....
Except the 1911 Census that is, when he has suddenly "gained" a middle name, his father's !.
We cannot work out why at all, it wasn't in memory for example of his "recently-departed" father as in fact he lived another 20 years afterwards, so ??. Maybe to avoid confusion with another man locally with the same name, now he'd moved from a quiet Forest village down to busy Newport ?. Perhaps it was an attempt to repair broken family ties/quarrels due to this move, he was only 22 at the time ?. However this seems unlikely, as a few years later his WW1 records dont show this new name, nor does his 1915 marriage, which was back in his home village with father attending.
We just don't know why he did it, but I guess he wasn't the only one who did.