In QGIS 2.18.18 on Windows 10, I have downloaded the NASA file srtm_05_01.tif that is available at http://dwtkns.com/srtm. I use the Terrain Analysis Slope method on that file with the following dialog box values
I use a z factor of 111,120 because the CRS is WGS84 EPSG:4326. The resulting raster file has the following histogram
where all degrees have 250,000 as a count. When I use the resulting raster in the raster calculator and select for an evenness of < 5 (i.e. "slope@1" < 5) I get the same output as selecting for an evenness < 87.
If this is not the expected behaviour, or if the srtm_05_01 file is unusual in some way, can someone suggest a file that would work well with the slope method?