It's not unusual among my friends. Most of us do this. I also often see the little gear switch looking thing on posts, which indicates only certain friends can see it. It's a good way to keep professional and personal contacts separate. Half of my friends are set to restricted so that they only see public posts, so I throw some out there on occasion so they don't get suspicious.
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usage imho to separate into lists and to post family updates to family, friend updates to friends, and keep some of your personal rants away from some of your work/professional contacts. But even my amongst friends who have more professional professions (eg. teachers) I find it unusual behavior to switch between the two. Being a professional/having a profession like teaching, does not automatically make someone FB savvy, and I think if you surveyed FB users you'd find that a huge number of them don't know where the gear switch is and don't know how to tell when their friends posts are set to public vs a list just by looking at the post in their newsfeed.
Then, when I look around FB, I see that most people will either set 'anything' to public, or they will have a wholly private profile with just the updating of the profile picture being set to 'public' because that is usually automatically set to public and less likely for anyone to go out of their way and purposely set it to 'friends only'.
Most people are not FB savvy imho. That is just an observation. Most people are not particularly internet savvy, and in the past ten years there's been a massive switch between those who were more likely desktop/laptop users and the influx of people using their smartphone to access the internet, and I think the latter group especially are less likely to be FB savvy, and the mobile platform is a bit more awkward to target to lists in my experience. In the pre-smartphone era I would occasionally come across, mostly older users, who didn't know about browsers or tabs or how to use the 'x' button, but a lot of people were more likely to explore what a computer can do and learn a bit more about the capabilities of the machine via the OS. Something like Win XP was wonderful for computer people, but Microsoft is designing platforms to be more user-friendly so you don't have to learn about the OS in order to use the internet. And there are exceptions to rules such as teachers making angst-ridden posts public and computer geniuses who don't know much about FB settings.
I would say you probably have a very FB-savvy set of friends, and that it's unusual to switch in that way, but having an FB-savvy set of friends, especially containing a high number of 'professional' people who are highly computer-literate, then there might be a bit of a viral effect of them teaching each other and compounding the difference between them and the general population of FB users who will use one setting or the other and will let devices either auto-set or inherit.
If we don't have access to statistics on this subject, then I guess it's going to have to be an agree to disagree matter, and a lot of our opinion on the subject will come from personal observations which will naturally vary between us all.