Are you asking why .40 S&W P99 doesn't have the metal machined away in the front?
If so, that has to deal with weight and slide velocity. When semi-auto pistols function, it's sort of a ballet of moving parts. In part, the slide's weight determines how fast it travels rearward and forward when the action is cycled by a bullet going off. If the slide is too light, it will move too fast and you will not be able to reliably eject spent casings. -You wouldn't want a series of stove-pipe jams. If you were to examine the .40 S&W variant of the Browning Hi-Power you would find that it too has a little extra weight and is slightly larger so that it will cycle reliably.
If you want to look at a gun where this has really come into play in a very noticeable way, check out the Glock 37. You'll see a slide that is slightly oversized and scalloped.