I've spent some time thinking about this before, and it provides a clue about the writers accent. For example:
"An 'H'", and "a 'H'" are both equally valid following this rule (by my interpretation), depending on how you pronounce the letter 'H'.
kichimi · · focus · HN ↗
"An 'H'", and "a 'H'" are both equally valid following this rule (by my interpretation), depending on how you pronounce the letter 'H'.
nemonemo · · focus · HN ↗