Posta are body positions. They provide a convenient way to talk about options and things to do. In general you should move from a posta, through a posta and to posta. If you're attacking, then you will almost always cut through Posta Longa. If you're covering (ie defending), then you're most likely going through Posta Frontale. However, you could also find yourself going through Posta Breve just as easily.
The wards themselves are not exact positions. There are variations of Posta di Dona, in particular, throughout the sword in two hands sections of all the manuals. There are key ingredients to distinguishing the poste from each other, and determining the tactical abilities of each wards, but in general, they aren't to be taken exactly as the position pictured.
There is no explanation within Fiore as to why he names some wards Pulsativa, others Instabile, and yet others Stabile. One theory is that you start with Pulsativa, move through Instabile, and end up in a Stabile Posta. The exception to this is Finestra, which is listed between the Pulsativa Posta. One explanation is that this is a ward you end up in when you draw your blade.
However there is another explanation that doesn't appear to have any exceptions. The designation refers to the blade and it's relationship or attitude to the line. This has nothing to do with your attitude/relationship towards the line, or your attitude towards anything. You can decide to make an instable ward stable on the line. Your decisions and attitudes don't affect the poste's attitude however.
Think of the poste as Dennis the Menace, the line as Mr. Wilson, and Dennis's parents, Mrs. Wilson and everyone else in the Dennis the Menace world as various governing factors of a fight. Dennis's attitude towards Mr. Wilson is that Mr. Wilson exists for Dennis's amusement and prank victim. At times the parents will influence Dennis to not annoy Mr. Wilson. That influence of behavior doesn't change Dennis's attitude towards Mr. Wilson. In fact there is nothing that will change Dennis's attitude towards Mr. Wilson, there are lots of things that change his behavior towards him however.
This is the same with the Poste and their attitude towards the line. There are things that will influence what they do, but not their attitude.