[mst]

The time of focus is not necessarily the time when the shutter button is pressed. In fact, there might be some delay in between, and at least in case of P and Tv modes, the aperture might change inbetween. Imagine you focus and then recompose...

You could of course suggest instead that the camera refocuses when you finally press the shutter button. However, that would probably lead to some sort of shutter lag.

-- Markus