For me, it only means "once in each fourteen-day period". It's not necessarily on the same day of each of the two weeks. If you break a month down into four weeks, it could happen once in week 1 (that's once in the first two weeks) and then happen again once in week 4 (that's once in the second set of two weeks). If it happens in week 1 and week 4, then it didn't happen "every other week".