It seems you can't actually download the original rainfall data, and all data available can only be used for comparing cells in a given image or detecting trends for specific regions between data sets.
In "standard" data types (as they call them) such as JPEG and PNG, "Dark blue areas show where a lot of rain has fallen. Pale green to turquoise show increasing amounts of rainfall". The data is scaled to a range of 0-255, but these are not actual rainfall values.
In addition, as mentioned here, other formats such as CSV are available, but these are also scaled and do not represent real values. NEO claim they can assist you in identifying the data source, so perhaps you should contact them.