There was a recent question on the Alteryx forum: How to use the percentile in summarize. The question misunderstands the percentile function in the summarize and is looking for something slightly different, although with similar math. So what does the percentile in the Summarize tool do? From the help:
Percentile: Calculates the specified percentile value for the group. The percentile is calculated by sorting the data and returning the row value relative to the specified percentile and its position in the sorted array – the largest value is the 100th percentile, lowest value is the 0 percentile, median is the 50th percentile, the 25th percentile is the value in the middle of the median and minimum, etc.