You didn’t read the specs it seems. It is explained in the specs on the Fairphone shop as follows:-
As mentioned it has been explained to other users.
Thanks to pixel binning technology, our new high-definition 48-megapixel sensor is always ready - even for low-light scenes.
Here, four pixels are combined to form one large superpixel that captures four times more light while being friendly to your phone’s storage (equalling 12-megapixel photos).