They are very individual animals and no one currently knows why they change their tentacles shape from long to short or bubbled to not. There are many theories floating out there but only that, theories.
I have 6 and they are all different, and they all started from the same one lol.
It is pretty easy to tell a healthy BTA from an unhealthy one and that is honestly all I worry about. Keep them in good params and out of my fans =D