Can someone explain why the value of a post fluctuates without a change to voters?

I've seen my latest storyhttps://steemit.com/story/@bobbybillbob/tham-chang-cave-laos-part-4
fluctuate in value while the number of voters has remained (19 as of this post). It hasn't been by more than a dollar, and usually a few cents, but can someone help explain the mechanics of the changing evaluation?

Thanks in advance!
-Bobby