Understanding Discrepancies in MAU Reporting Methods

What is the correct way to query your MAU? I have tried 3 ways of report setup and they have different results. Why is it?

1.
When breaking down by month, selecting Unique customer and MAU are slightly different (1732 vs 1726)
2.
Average number of line graph breaking down by month is different from selecting Metrics (1726 vs 1646)