One of the functions of the saddle is to set the height of the strings above the fingerboard (string action).
If you look at most classical or flamenco guitars, you will see that the saddle slopes from bass side to treble side – sometimes difficult to see clearly because the top of the bridge in which the saddle sits is itself sloping from bass to treble.
The slope is there because the bass strings need to be higher off the fingerboard than do the treble strings if they are not to hit the frets and buzz.
While this general rule has validity, you often see saddles where the slope is not a straight line down from bass to treble; instead it rises (or at least maintains a constant height) from low string E6 to d4 and then falls away to e1. What you have is a saddle whose apex is arched, low at E6, higher at d4 and then lower still at e1.
Why is this?
The reason seems to be – and I do not claim to fully understand why – that the d4 string can be slightly more prone to buzzing than the lower, A5 and E6 strings. Certainly when a customer asks me to set up a guitar with the lowest possible string action without buzzing then I am always tempted to create a saddle with an arched profile from end-to-end.