Welcome to Android Central! There will always be various other factors that might affect your battery usage, one of them being your cell signal strength. If your cell signal is poor, the battery will drain faster. (It doesn't matter if you're using wi-fi or mobile data to watch the video -- poor cell signal will still affect battery life, even while the phone is idle.) It's also hard to say if, during that 15 minutes, some other apps were active in the background (e.g., the Play Store auto-updating an app, or Facebook refreshing data).
The difference between 1% and 2% over 15 minutes is too small to determine if it's significant. What's more important is how your battery is over most of a single charge (i.e. from 100% down to about 20-30% -- don't make a habit of discharging it to near 0%).