Is it normal i got time off?

I work very hard at work and when their is an extra job to do I'll always volunteer. I often do a lot of extra stuff to help out coworkers as well. There was an extra optional event that work hosted that I offered to help with but cancelled last minute and asked if someone can take my place. This was approved however three other workers tried this and got this denied. What does this mean? Why would they only let me go? I usually sign up for hosted events as well even though it's not entirely required.