The Journey North map doesn't display the primary breeding areas accurately either. Example: it shows no breeding in northwestern Mexico, eastern Arizona or the Rocky Mountain Areas even though summer breeding occurs in those areas reliably every year (e.g. 1,400 wild caught monarchs were tagged in eastern Arizona last year). But it does show central Alberta as a primary breeding area even though monarchs don't occur there reliably every year.
I find it puzzling why Journey North would not make a map based on actual occurrence records like this: http://www.butterfliesandmoths.org/species?l=1892