I spoke to a Registrar in London some years ago who said that this sort of late registration was quite common at one time. Whatever the law said about registering births immediately, people didn't always bother. Especially if they were illegitimate, but sometimes for other reasons too. Some evidently just overlooked it, especially if you lived in a remote area far from the nearest registrar. He said this pattern changed in 1948 when child benefit was introduced. If the birth wasn't registered, you didn't get the money, so since then people have been much more meticulous about registering births, but prior to that clearly not every birth got registered.