If you're talking about daily rewards, click on the notification. It'll show you the rewards breakdown. I'm guessing your notification is covering rewards across several days.

When you have multiple rewards notifications next to each other (you had no other notifications between the rewards notifications), we combine them into one notification.

10% on comment zaps are taken to be redistributed as rewards.. as for post zaps.. no idea, received much less than indicated for that too.

If you are zapping content then that would decrease your balance.