Aei-our. Why does it have such a strange title?

The title lists six letters: a, e, i, o, u, and r.

The letters a, e, and i are the dominant letters for expressing front vowels. Front vowels are articulated in the front of the mouth.

The letters o and u, often in combination with r, usually express back vowels, so called because they are formed in the back of the mouth.

Front vowels are in red; back vowels in blue.

The hyphen in "Aei-our" also separates the front vowels from the back.

Similarly, in most lines in Aei-our, a region of front vowels is followed by a region of back vowels.

Therefore, it's "a, e, i, o, u, and sometimes r," rather than "a, e, i, o, u, and sometimes y."