Because when GTX isn't used, it do not work therefore no heat it emits. Browsing internet or watching YouTube don't need GTX to work. Optimus is software which makes intel hd graphic work on simple tasks and uses GTX for 3D graphics
edit:
Ok I see you were talking about no temp info. Maybe new GPU-Z have issues with 880M, on 760M it's working properly
STATION WAGON PC:
CPU: AMD Phenom II X6 1100T BE / A10 7700K MOBO: ASUS M4A79XTD EVO / ASUS Crossblade Ranger
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