There are a number of explanations possible - none of which can be 100% confirmed from the index entry on its own. You need to get both the certificates and compare them, especially looking at who the informants were, and from that you can work out the marital status being claimed.
It could indicate some change in circumstances between the two events, but it is also possible the information given was incorrect (on purpose or by accident), or possibly an error made in the process of sending the copied information to GRO and indexing it.
One possibility would be that the younger brother had a different father and the mother claimed she was single and was hiding the fact she was married to someone else.
When did the parents marry, and did the father die, or the marriage split up, between 1916 and 1919 ?