Yes, this is what I call "eco-tuning". Power draw scales aproximately with voltage squared. Close to the maximum boost every single bin of +13 MHz also increases the voltage by a bin of 0.013 V. This may not sound much, but your numbers fit:

Going from 1310 to 1265 MHz means stepping down 3.5 bins. Note the clock speds reported are not always accurate and may include some rounding, since Maxwells core clock can change so quickly. Let's assume 4 bins. This results in a voltage drop from 1.20 V to an expected 1.148 V - just as you're seeing.

Power draw will be reduced by 1.2^2/1.148^2 = 1.093, i.e. 9.3%. There's a further power reduction by 1310/1265 = 3.6% due to the clock frequency decrease. Overall power reduction is expected to be 13.2% (assuming no substantial temperature drop - otherwise there's a small further reduction), for a performance drop less than 3.6%.

Assuming a base power target of your GTX980 of 180 W (pretty usual), with a 105% modifier (189 W) would mean an expected power drop to 167 W, i.e. savings of 22 W. Factoring in a PSU efficiency of 85% (80+ Bronze) the power draw from the wall would be expected to change from 189W/0.85 = 222.4 W to 196.4 W, i.e. savings at the wall of 26 W. Add 1-2 W for the lower GPU fan speed and a few Ws saved from cooler components overall, and we're talking about ~30 W.

Which reasonably close to the 45 W you measured :)

(although realistically I'm probably missing something, since my estimate is off by a factor of 1.5...)

MrS
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