I believe that it is the angle from which the photos are taken and, probably, that both earrings in your first photo are not positioned entirely flat which creates the impression that the stones are 'irregular' in shape. Light refraction also has to be taken into consideration.
In the photos published in Roberts (page 59) the two top stones in each earring look round. Yet, they are not.
It is very obvious in all photos that the two pear shaped diamonds in the drops are not a perfectly matched pair.
The fashion for wearing earrings from a stud held against the ear lobe is very 20th century but, although Queen Victoria wore the earrings suspended from a hook, I don't think the composition of the stones has changed.