Cows walking through fences.

So I have a farm in my SSP world. It is just cows. Everyday in minecraft after I wake up from my bed, I go outside to check on my cows and apparently they somehow escape. I keep my fence-gate closed always. So the past few mc days i've been watching my cows, and then I see my cow just walk right through the fence when it's two stacked high! I don't kow how on why they do this. I think it is a bug. It's been going on in 1.4.5 and now 1.4.6, please can you try and fix this?