It's probably a combo of 1 and 3.
Sometimes you just forget or are distracted and they stay on.
Most of the time they go on when you think you may experience turbulence, e.g. near CBs, or other convective cloud. Sometimes it's on knowing that if you hit any of the clouds around you then it will be bumpy however you do a good job and don't actually hit one, so now you have the seatbelts on with no actual turbulence. Sometimes out of nowhere it gets bumpy, and even though you have periods of smooth air every time you go turn the seatbelts off you hit another bump, so you wait again.
See how it gets a little grey as to when you have them on or not?