'Hank' is an American variation, 'Harry' more of an English one, as far as I know. The origins of such variants are normally in phonetic simplification. The difficult double consonant 'nr' in 'Henry' gets worn down to 'rr', eliding the 'n'. Here, the 'r' is the stronger sound and is more likely to survive the elision. In another language with a less strong 'r', it could have been conceivable for the simplification either not to occur or for the outcome to be 'nn' for 'Henny' (which sounds like baby speak in English).
'Hank' is an American variation, 'Harry' more of an English one, as far as I know. The origins of such variants are normally in phonetic simplification. The difficult double consonant 'nr' in 'Henry' gets worn down to 'rr', eliding the 'n'. Here, the 'r' is the stronger sound and is more likely to survive the elision. In another language with a less strong 'r', it could have been conceivable for the simplification either not to occur or for the outcome to be 'nn' for 'Henny' (which sounds like baby speak in English).