Mobs displacing in trap

In 1x1 trap mobs are «displacing», «escaping» from trap even when not so many mobs in there.

Here videos:

http://www.youtube.com/watch?v=4nYTLD6O1IM&feature=youtu.be

http://www.youtube.com/watch?v=bJux80B2X7I&feature=youtu.be