Just varying the difficulty adds difficulty

I played against an AI in a game of Reversi/Othello over the weekend. It had three levels of difficulty, and once I'd played for a while I could easily beat each of the three levels of difficulty. There was a distinct difference of difficulty amongst those three levels, by the way: the first level was utterly trivial, and the third mildly challenging.

Then I tried something ... before each game, I would change the difficulty level, either up or down, no particular rhyme or reason. The same three difficulty levels now became a whole lot harder, and I found myself actually sometimes losing on the third level.

Read into that what you will.