As of the writing of this comment, your Steem Power was last changed 4 hours ago.
Basically, his is because Steemit will decrease it's donation to you, as the amount of SP you have goes up, and it recently decreased the amount that it is donating to you.
In more technical terms, @steem will match the amount of Steem Power you have in your account, so you have 15 SP. @steem doesn't instantly change it's delegation though, so for a short amount of time, you had a little bit more. But then @steem lowered it's delegation, so that it's delegation + your SP = 15 SP.
RE: Does steempower decrease on its own and why ?