Would you care to explain why? (provided that when we speak of battery life, we mean how much time it can go without recharging, and not after how many years you have to change it)
Because when you install a new OS there's things that are optimized in the background, and you also tend to use it more because of the "new and shiny" factor.
1. Do you charge it to 100% and live it alone until charge goes to 0%?
2. Do you unplug it in the morning and use it "normally" and see what charge will be left when you put it again on a charger when you go to sleep?
Different use cases drains battery at a different rate and its hard to use a new phone/a phone with a new OS "normally" because more time than usual will go into trying to understand it.
It may also happen that you start using the phone at a time when your phone usable is above your average.
I have a daily routine, always charging my phone at the same time, always using it at the same time. If after an update I notice the day after my battery is drained one hour before the usual time, it sounds pretty safe to assume the update did something regarding it.
But I won't decide I know for sure it's a problem for everybody, obviously (which doesn't mean either it isn't).
EDIT: but as philjohn mentioned, you can expect to use your phone more after an update (especially a major release) to try new features and because of renewed interest.