The 980 Ti still runs GPU Boost 2.0 which allows the GPU to hit a thermal limit of 80 C while overclocking it to the maximum that it will do within that temperature limit while under load.
My 760 has it too. Example: The clockspeed on my card is 1072 MHz, the boost clock is 1137 MHz. But with GPU Boost, it still overclocks itself to 1202MHz while staying within the 80 C (with margin of error, it sometimes climbs up to 82 maximum) limit at around 60% fanspeed.
Of course, with two of them in SLI the top card is always going to run warmer cause it draws in the used warm air from the bottom card (as hot air rises), especially when you have non reference models like yours.
Don't worry though, the maximum safe GPU temperature according to the Nvidia website is 92 C, so you still have a 12% buffer, which you'll probably never use cause as soon as it starts getting hotter than ~80 C, GPU Boost will reduce the overclock so that the card can cool down a bit again and stay under, or at that 80C mark.