Average Slope of Line using Add Surface Information

[AvidTrails]

I work a lot with slope lines, specifically of trails and roadways. I just used Add Surface Information to calculate the slope of a line, using a raster. 

The lines low point is 5409 FT. The high point is 5294 FT. The line is 3190 LF. Basic rise/run math would tell me this is a slope of 3.6. Add surface information is telling me the average slope is 6.5. This is happening in a variety of instances. Why would Add Surface Information not be yielding the same value? 

All data is projected to the same coordinate system, which is NAD 1983 StatePlane Utah Central FIPS 4302 (US Feet)

[DanPatterson]

apparently the low point and high point aren't at the start and end of the line, therefore you line length needs to be determined between those two points

Addendum

It would also be better to determine the sequential differences to obtain an overall average

How To: Calculate the Slope of a Line Feature across a Surface in ArcGIS Pro (esri.com)