[greenshackle2]

They're pronounced differently:

History: /ˈhist(ə)rē/

Historic: /hiˈstôrik/

The 'h' is stressed in 'history', not in 'historic'. Try pronouncing 'historic' with a stress on the first syllable, it sounds wrong. Since it's softer in historic it's more natural (to me anyway) to use 'an'.

(Also, I'm french so I barely pronounce h's to begin with - so in my speech the stressed 'h' in history is kinda soft, and the unstressed h in historic is barely there.)

(edited)

[stretchwithme]

I've never heard the Hs pronounced differently here in the US. Perhaps its too subtle for me.