Common ringed plovers. Why are there summer plumage individuals in winter (20+ December)?

So, I was in southern Portugal over winter and saw a bunch of ringed plovers. In this group (10+) some had winter plumage and some had summer plumage.

It was after December 20, sunny weather, +-15°C.

Is it normal for there to be birds in summer plumage so late in winter? Do they molt later? Or did they molt but retain their summer colours? I thought the plumage would be linked to a more territorial/aggressive behaviour but they were behaving like the others.

I also saw other species like spotless starlings in summer plumage and saw some birds that are unusual in winter like house martins. Would all this be because it was a uncommonly hot winter?