I think it's not <that> uncommon. Were they older? I think how it goes down is that women don't NEED the sexual component to the relationship as much as men esp post menopause so then both of them are like finally, we can just chill out and enjoy each other without really having to concern ourselves with this aspect of it.
"Lesbian bed death" is already an aspect of female relationships that the ladies have to contend with, as estrogen led partnerships often end up with both people just feeling really complacent about the whole thing. Neither wants to initiate, both want to feel desired blah blah and over time, they are having sex less and less.
Plus the normal slope that comes with being with someone for a while after the New Relationship Energy wears off and you have a real recipe for not having much need for sex.
I guess as long as both people are cool with it then it doesn't matter. I'm sure that sometimes they hit these various milestones at different points and that must suck, just like heterosexual relationships. Creating a dynamic of frustration and all that other stuff.