The renders use LGEO, and since LGEO bricks are custom POV code (completely ignoring the original LDraw geometry), they can include whatever surface texture they desire on whatever surface.
The implementation details for LGEO are that "slope" bricks are tagged as such in the lookup table that maps from LDraw to LGEO. Any bricks that include textured sloped geometry are split into two separate POV objects, one that contains the sloped geometry, and a second one that contains all the other geometry. Both objects are then referenced from the parent object with the same transformation matrix, but with different textures. (In POV, "texture" includes color, image-based or computational texture graphics, image-based or computational bump maps, and probably other things.)
If you look at the POV output from LDView or L3P when LGEO is enabled, you will see these two entries for sloped bricks. (For example, LDView's m6459 sample model has two references to lg_32083, and two to lg_32083_slope, because it uses LDraw part 32083 twice.) To enable LGEO output in LDView's POV export, check the "Use XML mapping file" option in the "Native POV Geometry" group of its POV export settings. Leave the "Path to XML mapping file" field blank to use LDView's default LGEO mapping file.
The implementation details for LGEO are that "slope" bricks are tagged as such in the lookup table that maps from LDraw to LGEO. Any bricks that include textured sloped geometry are split into two separate POV objects, one that contains the sloped geometry, and a second one that contains all the other geometry. Both objects are then referenced from the parent object with the same transformation matrix, but with different textures. (In POV, "texture" includes color, image-based or computational texture graphics, image-based or computational bump maps, and probably other things.)
If you look at the POV output from LDView or L3P when LGEO is enabled, you will see these two entries for sloped bricks. (For example, LDView's m6459 sample model has two references to lg_32083, and two to lg_32083_slope, because it uses LDraw part 32083 twice.) To enable LGEO output in LDView's POV export, check the "Use XML mapping file" option in the "Native POV Geometry" group of its POV export settings. Leave the "Path to XML mapping file" field blank to use LDView's default LGEO mapping file.