I'll add this: When you take the polls as a whole, it seems to me that the Conservatives' numbers are where most of the disagreement among the all the pollsters, i.e., not just Nanos, is. If the shy Tory effect is playing a role in the Nanos numbers, and my theory that incumbency is worth a couple of points that I've floated around based on polling from 2004-2011 is accurate, Harper might surprise again like he did last time around.
https://en.wikipedia.org/wiki/Opinion_polling_in_the_Canadian_federal_election,_2004
https://en.wikipedia.org/wiki/Opinion_polling_in_the_Canadian_federal_election,_2006
https://en.wikipedia.org/wiki/Opinion_polling_in_the_Canadian_federal_election,_2008
https://en.wikipedia.org/wiki/Opinion_polling_in_the_Canadian_federal_election,_2011
It might be worth adding that while the Liberals were underestimated in 2004 and 2006, the Conservatives appear to have been slightly more so in 2008 and 2011. That may support your theory more. If it does, should we expect it to be more pronounced this time?