This also gives us insight into why the distance may be in miles. Surveyors generated information that resulted in maps using miles for land based areas including the coastline. They then apparently just continued surveying across the lakes to complete the survey.
I suspect current opinion is that triangulation using a Mercator chart except at high latitudes results in minimal error. And navigating at the latitudes of the Great Lakes using Mercator charts does not introduce large errors. So I suspect Mercator charts probably will replace the Polyconic charts in the Great Lakes as time goes on. Already, all large scale (numerical) charts are Mercator except Lake Huron. Lake Erie was converted from Polyconic to Mercator sometime in the last 30 years or so. However, until all small scale (numerical) charts convert over or at least add nautical mile as a scale, it is unlikely that the nautical mile will become the predominant unit of measure in the near future.
I have read some really interresting opinions as to why Great Lakes boaters use statute miles and mph.
Hopefully this gives others insight into why many Great Lakes Boaters navigate in statute miles.