Try thinking of it like this:
Each value (in this case the seed you chose) is a point on the perimeter of a circle. By selecting a "seed" you are starting in a specific spot. and by advancing the seed you are moving to the next consecutive value along the circle. The values are not arranged in order (0, 1, 2...) as you move around the circle. They appear random when you only see a few frames (when advancing the seed following the algorithm} but every value from 0 to x( where x is determined by the precision of the algorithm) exists on the crircle. 1 byte precision allows for 256 points on the circle 2 bytes allows 65536, etc. In SWSH the algorithm has 8 byte precision. which allows for 18 quintilion points.
What kaphotics means when he says "they advance in a predictable fashion" he means, if you know the algorithm you can predict the next vslue if you know the current one.
I hope this helps.