This retaining wall idea is very simple in both design and elements. Honestly, compared to the rest of the front yard design, not much thought went into creating this specific shape of the flower bed and retaining wall. However, there is still some reasoning behind it.
The shape of the lower flower bed could have been almost anything as long as it was curves. Even a half circle would look fine as a variation. The wall itself curves inward for retaining strength as well as design.
The different color brown rock adds a visible defining edge for the planting area. It also adds different texture and color to the list of elements to design with.
The shape and curve of the wall just feel the natural choice for this design. In retaining and terracing a slope this small you could possibly reverse the curve without needing more structural support. However, anything higher, and the inward curve as in this retaining wall will give more support for terracing the slight slope.
To change this design and reverse the curve of the front yard retaining wall would give it an entirely different feel and look. It would actually be quite appealing and give the opportunity to create even different planting spaces below. It's just a thought if you might like a little variation.
You may also want to see landscape design on a slope or hillside for several more examples from other designers.