whitebox:wbt_hillshade Why are values not between 0 and 255 in R?

Question

If you follow the tutorial here: https://vt-hydroinformatics.github.io/rgeoraster.html

I want to create a hillshade map. In general hillshade is between 0 and 255 but these values are very high. How do I interpret them?

wbt_hillshade(dem = "McDonaldHollowDEM/brushDEMsm_5m_crs.tif",
              output = "McDonaldHollowDEM/brush_hillshade.tif",
              azimuth = 115)

hillshade <- raster("McDonaldHollowDEM/brush_hillshade.tif")

tm_shape(hillshade)+
  tm_raster(style = "cont", palette = "-Greys", legend.show = T)+
  tm_scale_bar()