Maybe it's an accident. In those days coins are minted by hand, so someone might've struck them over a hollow object, causing the curved shape. I'm not an expert but what i do know is that the shape of the coins will decrease it's value.
These are real, and were not shaped like this by accident. Coins of the Himyarite Kingdom are often scyphate (“Bowl” shaped). This was done intentionally during the minting process, similar to the scyphate coins of ancient Samarqand or the trachies of the Byzantine Empire. We don’t know exactly why they did it, but it may have had something to do with the coin’s durability. This shape does not decrease the value of the coins, as it is part of the original striking process
If you're referring to the purple, that's part of the legend. If you're referring to the red, that's a scepter symbol likely used as a dynastic identifier on Himyaritic coinage
