The simple and real reason the tablets of the 10 commandments commonly appear rounded on their tops, like two semicircles, relates to historical factors. Back when Jewish books had to pass through Gentile censorship, they had to be printed by Gentile publishers who printed this image on the first page of Jewish publications. Jews paid the matter little attention, but this 1st-page custom propagated from generation to generation.
The fact is this image of the tablets rounded at their tops contradicts what the Gemora (B. Basra 14a) tells us. There it states clearly the tablets' dimensions and that they filled, with two other items, the hollow of the ark - in such a way that no empty space remained therein. The tablets were 6 x 6 x 3 (tefachim) and flat on all sides (for rounded edges would leave unused, empty space).
A book by a rabbi a few decades ago, published in Israel, proved the accuracy of this Gemora. Nevertheless, the rounded design remains in prominent display in far too many synagogues across the world, perpetuating the error - one that stands in contradiction to what our cherished Gemora teaches.
Besides in synagogues, they abound as well in educational institutions where young children learn in their formative years. This inaccuracy embellishes Jewish flyers, receipts or school certificates. Jews thus have the wrong impression and take for granted this erroneous appearance.
The source of this mistake derives from Gentile persuasion and history. It contrasts with what Gemora teaches. When the tablets are rendered rounded, it enhances the non-Jewish version over the words of our venerable sages.
Similarly, this same perverted phenomenon holds true for the shape of the Temple Menorah, which too has a history of distortion, as written
here.
Here, for example, is a picture of this distortion, in the famous Hurva Synagogue in The Old City of Jerusalem, which, in these very days was again inaugurated (for it had been destroyed and rebuilt a few times). This same distortion can be seen even in most orthodox synagogues around the world.
How to explain it? People just don't know.