Something I've had happen twice today (not in the snapshot, 1.3.1) while I was breeding my cows. I started with 8 cows, each one ready to breed. Gave wheat to all eight cows, expecting to see 4 baby calfs. To my surprise, I somehow got more than I bargined for! Once I got 5 baby calfs from breeding eight cows and 6 baby calfs from another surprise! Is it possible for breeding to give "twins," or do I have a "player" in my pen mating with more than one other cow?
UPDATE: This wasn't a miscount on my part. It just happened again and I've figured out what might be causing it.
I usually start by having the wheat out for only a split-second so that I don't get crowded by the animals I'm breeding. Then, if you're quick on the draw with the wheat and give wheat to all of the animals, the animals apparently have an "orgy" and group up. With proper positioning, the breeding "partners" also bump into other "partners" and cause the extra babies to spawn!
This is my current theory, no video just yet as I need to tweak settings with FRAPS so that I get the proper rendering.