To me it seems it is more a 4-4-1-1 or when Cahill does drop back a 4-2-3-1. Granted I could very well be wrong as I have a relatively young football mind. I will agree that in the last two matches it is Saha who has dropped back more than Cahill. We have gotten 2 wins in the last 2 with that formation so I can't argue with the results.
Well, you're right in that one of them usually drops off, either Saha to get the ball or Cahill when we are defending.
But that would happen if we had 2 (in name) strikers playing, or at least it should do. That's just natural movement of the team during a game.
Otherwise, if the listed formation was based on where players go during a game we'd be playing 3-5-2 or 3-6-1 given how much time Baines is in the opposition half.