Ohh it may because of the voting power (I'm not sure with the term), which means that the more Steem Power a person has, the higher the $ he gives you by upvoting ur posts. The case may be that yesterday, some people with larger Steem Power voted u. However, although you have more votes today, the ppl who voted you aren't "strong" enough. I'm not sure if I'm correct but it's usually the case :) hope it answers you!