Are there any buf_3D featureclasses created? This is the actual result of the tool and not the pnt_buf featureclasses. If these are not created the process is not running completely and some error is occurring in the process.
If I select a single line that passes through some relatively steep slopes (line Line_D 357):
... and I extract the slope values, the mean slope is 25.7%:
This would mean that the buffer should pass at 2905 meters from the center point. It passes at 2920. The reason for this difference is that the slope direction is taken into account. If you traverse a steep slope maintaining the same altitude, you will traverse at a slope of 0%. A slope map returns the maximum slope in any direction..