You may see differences in your metrics when these are analyzed using specific dimensions. These differences only occur for metrics such as "Reach", "Audience", and "Exposed Users". When analyzing such metrics in comparison to time, advertiser, categories and campaigns, some instances may be counted more than once.
When analyzing your daily reach during the last 30 days, a single customer, that was reached more than once during this period, will be counted for every single day that he/she was reached. This represents your accurate daily reach. However, when you analyze your monthly reach, the same customer will only be counted once.