People love to use statistics for things like estimating how much ammo they will need in a gun fight. While it may be most common for folks to only need 2-3 rounds for a gun fight (mean, median, or mode), what has happened in past gun fights does not determine or give a percentage indication of what will happen in any new gun fight in which any of us participate.
Sure it does. It lets you know that there is a high percentage of probability that you will be able to solve the problem with a fairly low number of rounds. It tells us that we probably don't need to carry 100 rounds with us all the time. You might not be right in your decision about how much to carry, but the statistics have given you a pretty good idea about how many rounds you will neeed in a gunfight. Past gunfights don't determine what wil lhappen in a new gunfight, true, but they do give us an indication of what is likely to happen and what is unlikely.