Part 30099 - Brick Arch 1x5x4 Inverted doesn’t seem to fit into the LEGO system. I cannot find a part that fits on the bottom stud and provides continuity to the concave slope of the arch. A tile is not high enough, and a cheese slope is too high. In other words, the height is 1.5 plates (1/2 a brick). Does anyone know why this is and/or have an example of a LEGO piece that fits this slope?
I would assume that this is to do with stability and flexibility of the element, combined with the rectangular nature of the system.
There are a number of uses of the part where it's not used to create a full circle, such as on the tailplane of 31011 or the fairing of a Ferarri and having a stud at the bottom allows them to secure the element on the top and bottom, rather than just from the bottom.
If the element was a true quarter circle, the end still wouldn't line up with the system height correctly, so I imagine that the decision was taken to finish it early with a stud, rather than complete it neatly.
As you can see, arches form a nice circle to fit round modified plate. All fits well, except the very bottom. Making the bottom edge fit the plate/tile nicely would result in gap with a round piece. The curve of this arch brick isn't full 1/4 circle to make it flush with a tile/plate.
However you may ask "Why didn't LEGO just extend the bottom curve to full 1/4 circle and cover the stud?". This seems to be an obvious solution, but that's a different topic and we may not know which decision caused the part to appear in existing form.
Finding a piece that completes a flawless circle can't be done with this element's current design, but...
...If we stick only with Part #2339, we can build an outside frame that changes the direction of the studs themselves in a way that produces the desired effect.
You could try and contact LEGO customer service directly for their official reason as to why this element is designed in such a fashion as to not produce a perfect circle in conjunction with its inverted twin, because a question this specific could only be verified officially by LEGO themselves, and anything else would seem to be speculation.