That and the Panzer IV's glacis plate has a ridiculous LOS thickness. If you hit there, you WILL bounce. If hes angled, his 80mm drivers plate will stop most of the crap flying around.
Also, the Panzer IV F2 can kill a Panther in a few hits out to about 1600yds. I've personally done it before. I've one-shotted T-34's from the front with an M4(75) as well. Its all about shot placment. The lower hull of the T-34 is very squishy, as it is for the M4, and Panther, although the T-34 seems to be the squishiest. The reason is that the T-34 has 45mm of sloped armor, which curves around to the lower hull. As it curves, the slope decreases, so there is a point where you only have 45mm of protection.
The Panzer IV's armor, while less sloped in general, is much more angular, which means that it will give more random "wtf" moments than the T-34 will with its more uniform shape. My guess, you just had really s**t luck.