The two fonts have slightly different font metrics, which means that the distance from the top of the inline-box to the baseline and from the baseline to the bottom of the inline-box is different for the two fonts.
The fonts are aligned by their baselines so the top and bottom of their inline-boxes are slightly offset with respect to each other.
Because the height of the h1 block is the height of its contained line-box and the line-box must be large enough to encompass both the inline-boxes from the upper of the two inline-box tops to the lower of the two inline-box bottoms, the total line height is slightly greater than the line-heights of either font.
If you stop the two fonts from being aligned on their baselines: e.g. h1 font { vertical-align:top; }
you see that the height of the h1 drops to 48px.
See http://jsbin.com/hisihojozi/2/edit