For example, WhatToMine is estimating like $3.67/hr yet I just looked at my proceeds overnight (ETH price has been stable), and over the last 13 hours my mining proceeds have netted me $4.29/hr. That is a 14% difference from estimate to reality. $88/day versus $103/day.
Bitcoin gained ~10% today, so while the BTC/ETH pair might've been more or less stable, the BTC/USD pair wasn't. That's probably where most of your "14% difference" is coming from.