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.
The producer was supposed to be in charge. He tried to keep the artists out of the mix room. But something went out of control. The producer was gone for the day, or the bassist had a fit of megalomania. These things actually happen, even after weeks in the studio, and the problems sometimes don't become apparent until the album is assembled in its intended order, or auditioned in a good monitoring environment.