Probably because there is more visible light in the false color IR image.

  The B&W is tuned further into the IR.

  Jeff

 After all focus is focus.

Focus is focus but you are focusing something different. Can you focus
the TV signal on the camera?

Radio, Tv, sound and light are all the same thing at very different
frequencies.  IR, UV and Visible light are all very similar but
different.

Also the way the film reacts to the waves will be different depending on
chemical makeup

On my 35mm SLR there was a red mark just offset from the normal focusing
mark for IR film.  The manual may tell you where the one for your lens
is.