When counting sefiras haomer, we count the number of the day we are about to start but the number of the week we have most recently completed. For example, on the first day we say "today is one day" even though we have not yet completed the first day as it is just starting. On the other hand, when we count weeks, we don't count a week until we reach the end (e.g. "today is seven days which are one week"). What is the reason for this inconsistency?
To address the questions that have come up in the comments and answers so far, there are a number of ways the days and weeks could be counted consistently. For example, "Today is day 1, which is one day of the first week of the omer." Alternatively, as Double AA mentioned in comments, simply counting days and weeks separately: ״היום יום אחד לעומר. השבוע שבוע אחד לעומר״