The variation likely happens because of differences in the sample length. Imagine a long hill with an average 5° grade, but with a short section of 100 feet with a sudden rise and 10% grade.

If the grade is measured for only that short section the maximum grade reported will be that 10°, however depending how long each sample is and how that section was blended with others you'll get a lower number based on the blended 5 and 10° sections and how much of each type was included.