@dynamicrypto, I may be wrong but in my opinion Steem do not directly traded with the USD, and inturn the value is based upon equation of Steem = Bitcoin Rate and Bitcoin = USD Rate. So, i think that can be the reason while estimation it's directly compared with the USD rates but Steem don't have direct trade with USD so, while payout the calculations may be different. In my opinion more experienced people can put the clarity but i think this way. Stay blessed. 🙂