After conducting a one-to-many join, the statistics values for a number field increase when compared to the pre-join values.
In a one-to-many join, only the first matching record is joined and displayed in the layer's attribute table. However, when using some tools, the subsequent joined records are used in calculations.
This means that even though only the first record is joined and displayed, when calculating statistics for a field all values are included.
In ArcMap 10.1, when exporting or using the Feature Class To Feature Class conversion tool on a feature class with a 1:M join, the resulting output contains all values and not just the first matching record.