So, I've been playing my brother's Runescape account for a while; I don't like the game enough to invest that much time in it myself, but I really enjoy merchanting, and it makes my brother money.
Several months ago, I discovered a strange phenomenon: oak logs would alternate between ~19 gp and ~40 gp. As in, it would sit at 19 for a week or so, and then jump up to 40 for a week.

What I'm wondering is whether or not this sort of thing is normal in an MMORPG.
Has anyone else seen things like this?

And, does anyone know why this worked?