Fence Glitching

Several Times I was playing around with the "Chicken Jockeys" and
"jeb_" sheep, and they escaped from there Wooden Fences. No Fence gates.
So I made the fences one higher. After two more Chicken Jockeys escaping and one _jeb Sheep, I built an extra layer of 1 block high fences around the fence. (Only for the chicken jockies, so many of them had escaped but only one sheep)
Later I found Yet ANOTHER had escaped for a TWO block fence.
They seem to be jamming themselves into the corners. (The sheep rarely went into the corners so they only escaped once), but the Chicken Jockies have escaped many times. Unknown if this is bug/intended feature.