With pmim you can get a more oblique angle on the string, whereas when
using both i and a your attack will be either more of a right angle or
you will either be reaching with i or crouching with a.
Also remember, women generally have longer i fingers then men, so i and
m will be better matched.
Using two nails instead of three makes tonal consistency that much
easier to get.
Maybe she tried both when she was young and happened to pick this one up
faster, and just ran with it... Who knows?
--
Matt