Sanskrit builds a great deal of its vocabulary by raising a vowel through two fixed steps, which the grammarians call guna and vrddhi. The basic vowels i, u and the vocalic r each have a first grade and a second: i gives e and then ai, u gives o and then au, and the vocalic r gives ar and then ar with a long vowel. The system is regular enough that a reader who knows it can often see the underlying root inside a form that no longer shows it.
The letter table records this without saying so. The four vowels that close the vowel series, e, ai, o and au, are exactly the products of the operation, which is why they have no short partners and why they sit together at the end. Their arrangement is not a list of leftovers but two matched pairs, each pair being one vowel taken up two steps.
The point matters for reading Siddham because it makes the script partly predictable. Buddhist Sanskrit generates terms constantly by this route, and the second grade in particular marks the words that name a school, a doctrine, a class of being or a quality abstracted from something concrete. A reader who recognises the vowel at the front of a word already knows a good deal about what kind of word it is, before working out what its root means.