However since the lone pairs are 'invisible', the shape of ammonia is pyramidal . The two electrons are located between two nuclei, and are attracted by both. A lone pair is different. It is necessarily only attracted to one nucleus and the consequence is that it adopts a position effectively closer to that one nucleus than the bonding pairs of electrons. This means that the effective solid angle occupied by a lone pair is greater than that occupied by a bond pair.
Lone pairs demand greater angular room, and are located closer to their atoms than bond pairs.