I would like to add that you ALWAYS find those words in order.
For example, FUS RO DAH
you will always learn Fus first, then Ro, then Dah.
It doesn't matter which word wall you see first. You always learn first word first, second first, then the third.
I have no idea what the ingame explanation is. I mean the writing is written in the wall. However, it doesn't become permanent till you see it.
The ancient carvings on Word Walls can vary, and are dependent upon
what Words of Power the Dragonborn already knows at the time of the
encounter with the wall.
For example, if the Dragonborn were to learn the first Word of Power
for Animal Allegiance from Ancient's Ascent, then the wall would
display the first carving in the table below. However, if the
Dragonborn was to learn the first word from Angarvunde, then the first
carving would be displayed in that location instead.
If the Dragonborn was to later visit Ancient's Ascent to learn the
second word, the carving would be the second carving in the table as
the first word has already been learned.