The next step is the mixdown. This mixdown may take anywhere from 4 hours to 4 weeks, depending on the producer's predilections, the artist's whims, and the budget. Usually each tune is mixed in isolation. Rarely do you have the luxury to switch and compare the songs as you mix. Some mixes may be done at 2 o'clock in the morning, when ears are fatigued, and some at 12 noon, when ears are fresh.
Every mix sounds different, every tune has a different response curve.
The Skew of the Monitors Monitoring speakers. It's amazing when you think about it, but very few studios have accurate monitor systems. A typical control room is so filled with equipment that there's no room to place a monitor system without causing comb-filtering due to acoustic reflections. Usually there is no time to see how your music will sound on various systems in different acoustic environments. Some frequencies stand out too much, and others too little.