Ride with GPS provides a detailed explanation on the ways elevation (and resulting grade) is calculated. I found the answer in the help section HOME/SUPPORT / GRADE AND ELEVATION
WHY DO I SEE DISCREPANCIES IN ELEVATION BETWEEN A PLANNED ROUTE AND A RECORDED RIDE?
Essentially the grades you see on a planned route are based off mapping measurements taken from space. The grade number you see on your Garmin is based on GPS or altimeter sensor depending on the Garmin unit. Since there are so many variables with the Garmin, I tend to rely on the grade figures on a RWGPS planned route.
This is the online source for county maps I use along with the atlas