[OT] Why is the Khmer om sign called om and not um ?

[Patrick Andries]

 I was wondering why Unicode 4.0 refers to one of the dependent vowel signs composed with nikahit (aka "am" pp. 278-279) as "om" while the other one is "aam".   If "aam" has a name based on the other character used in the composite vowel sign (U+17B6 AA), an "etymological" name distant from its prononciation [ɔ́ɘm], why would not "om" be called "um" since it is composed with U+17BB whose value is U ?   This is incidentally closer to the transliteration in Daniels & Bright, p. 469 : « ʔum ». This is also the transliteration "uM" used on top of page 8 here   http://semioweb.msh-paris.fr/escom/projets_recherche/01_02_projet_inde/IEMCT%20PAPERS/Content%20Creation%20and%20Information%20Retrieval/Minegishi%20-%20Khmer%20and%20Thai%20Scripts.pdf     P. A.