The number represented is the percent deviation over the time span of the amount of Steem available. So -.87 would be a 0.87% drop in the amount of Steem due to people powering up more than powering down. If there was 72 million Steem on day one and 71 million Steem on day 30 the formula would be (71 - 72)/72 * 100 = -1.39 or a 1.39 percent drop in the supply of Steem.