ISWest calculates monthly bandwidth usage with an industry-standard method of calculation. Although it sounds somewhat complex, the 95th percentile is a relatively simple calculation that is used to evaluate the regular, sustained utilization of your network connection by filtering out the major spikes in your usage throughout the month.
One major point of confusion is the comparison of 95th percentile vs. average bandwidth usage. The 95th percentile is not an average. The 95th percentile represents the value that your bandwidth is at or below 95% of the time.
ISWest calculates 95th percentile by capturing bandwidth usage values every five minutes during the course of the month. At the end of the month, the top five percent of values are discarded to avoid penalizing clients for "short spikes" or atypical usage. After filtering the top five percent of bandwidth usage values, the next highest number represents your 95th percentile number, or your actual bandwidth usage.