A: The Attribute Manager does not use percentiles but is binning data into equal bins. The reported values for each percentile bin are the maximum values for each bin. For example, the dataset 1,2,3,4 split into two bins will have maxima of 2 and 4 yet the 50th percentile value is 2.5. The Summary Statistics utility uses an algorithm documented in http://commons.apache.org/proper/commons-math/apidocs/org/apache/commons/math3/stat/descriptive/rank/Percentile.html.
ioGAS uses the 2nd column in the following reference page to calculate the percentiles used in Summary Statistics.