Ammonia’s central atom (N) has one lone pair and three bond pairs.
In water, the central atom (O) has two lone pairs and two bond pairs.
As a result, the two bond pairs repel the two lone pairs on the O- atom in the water molecule. And the attraction between the lone pair and the bond pair on the O-atom of H2O is stronger than the attraction between the lone pair and the bond pair on the N-atom of NH3.
As a result, despite their distorted tetrahedral structure, H2O has a lower bond angle than NH3.