Is this the "3 arc second SRTM" data? Do you mean 90 because the approximate resolution is 90m?
Are you trying to verify whether Spatial Analyst produces the correct result?
I would say the problem lies in comparing the slope raster which was generated from the DEM itself and the contour lines which are too dense for this type of DEM. I'd say there is a certain degree of interpretation that goes on in the contouring algorithm to calculate the contours from the DEM this course.
Also you will probably have better luck posting this in the Spatial Analyst forum.