Undervotes in Oregon: Caused by Two Sided Ballots?

A fascinating conversation is going on here on undervotes in the recent Oregon referendum.  The undervote rate on Measure 67 exceeded the undervote rate on Measure 66 by as much as 1% in one county, and more than .75% in three counties.

A spreadsheet displaying the numbers can be found here.

The consensus is slowly emerging that the higher undervotes were directly correlated to the use of a two sided ballots in some, but not all, Oregon counties, which leads one to ask, why would an identical set of ballot measures with no local races appear one-sided in some counties and two sided in others?