To understand 'issues per unit' versus 'units per issue' you can read it as '<*issues* per unit> issue(s) every <*units* per issues> <unit>'. For example, a biweekly frequency (every two weeks) would be '1 issue every 2 weeks'. So 'issues per unit' would be 1 and 'units per issue' would be 2. See below for more examples.