As seen on Twitter: https://twitter.com/Dinnerbone/status/552263565668016128

The game appears to be able read the frames in the MCMeta file, but it doesn't recognize that the MCMeta file has specified the frames to be 64 by 48.

The animation table has 9 frames that are 64 pixels wide and 48 pixels long.

I assume this affects 1.6 and 1.7 as well.