COGNOS Percentile Function

[recklessonemb]

I am struggling to get COGNOS to report a 90th percentile value that is what I would expect:
I have a report that has a simple duration value in hours in each row (nulls filtered out) called the TPIA and a calculated field that is just simply percentile(TPIA)
The value it reports for a 90th percentile is very different than what Excel reports for the same dataset using the percentile.inc function and is also NOT 90% of the way down an ordered list of the values.
Can somebody explain why this might be?