Isanger, the answer given by Metrodus is correct. It is not necessarily the case that one Latin letter always corresponds to exactly a single sound. In this case, the single i actually stands in for a double i, meaning that the i makes both the preceding vowel a diphtong and serves as a consonant for the second syllable (so, e.g., ei-ius). From Allen and Greenough:
http://www.perseus.tufts.edu/hopper/tex ... smythp%3D6
Between consonant i and a preceding a, e, o or u, an i was developed as a transient sound, thus producing a diphthong ai , ei , etc., before the consonant i. In such cases but one i was written: as, âiō (for † ai-iō ), mâius (for † mai-ius ), pêius (for † pei-ius ).
Also note that having two vowels together that could
form a diphthong does not mean that in 100% of words that they are pronounced as a diphthong, even if that is usually a safe assumption.