Because you use baby fans. I use two 6 inch fans plus a large crossflow gutted from a full size floor fan. I also rely on evaporative cooling, a good thing for allowing plenty of kalk dripping.
Besides, I didnt say that an open T5 retro setup wouldnt run cooler. I said that my open frame halide and T5 set-up is easier to cool than an 8 bulb T5 FIXTURE
Just like I stated ... it's not just watts or type of lighting that determines heat issues, it's also implementation of the lighting and design of the fixture/canopy/frame etc.