Mercedes arrive at Monza clearly ahead of their counterparts, this hasn't stopped the team from introducing new components though, with a change made to their floor just ahead of the rear wheel for Monza.
Red Bull & McLaren adopted earlier in the season. As we know the area is critical to diffuser performance as airflow managed by these slots can increase the performance envelope of the diffuser.
NB: Tyre squirt is airflow that is laterally injected into the diffusers path by the rotation of the tyre, robbing the diffuser of performance. The area isn't subject to FIA load tests, so one can surmise that the dog legged design offers a favorable advantage in terms of flexing, when compared to a straight forward slot.
Furthermore, as this is something introduced by Red Bull, one has to think to the past to understand what advantage they are trying to recoup. If I look at how the dog-legged slots are designed I would also have to consider a design by Red Bull that was banned amid controversy in 2012. The dog legged slot allows for a more expansive hole ahead of the rear wheel (raised vertically), whilst the slot is presented much further forward, reducing the effect it has on the airflow passing through the hole. Which if compared to a slot that runs along the Y axis start to finish, the net effect will be lowered by the exposure at the periphery.
Worth noting is that although 3 teams now run this dog-leg design their interpretation will all vary, owing to other airflow structures being built in the area.
We must also consider that with the recent loss of FRIC means the car now behaves slightly differently at higher speeds, as FRIC was being used to regulate the ride height and therefore the rake and attitude of the diffuser. Adjusting the aerodynamic behavior of the floor is just another way of getting the optimum from the overall package.