Just to put some actual stats behind it:
Zeller and Varejao played 22 games together in '13, and Zeller averaged 22mpg with Varejao averaging 36mpg. In the 55 games without Varejao, Zeller averaged 28mpg.
Zeller and Varejao played 53 games together in '14, and Zeller averaged 13mpg with Varejao averaging 27mpg, but those include 15 games with Bynum at 20mpg, and 16 games with Hawes at 28mpg.
In the 24 games where Zeller and Varejao played without Hawes or Bynum, Zeller averaged 14mpg and Varejao averaged 32.
There were 7 games were Varejao, Bynum, and Hawes didn't play where Zeller did, and in those Zeller averaged 22mpg.
Maybe you can factor in different coaches in 2013 (Scott) and 2014 (Brown) too.