iOS 8: UITableViewCell.contentView doesn't resize subviews in it

I put an CustomView in UITableViewCell.contentView, setting the autoresize mask of the custom view as W+H.

But when running, the CustomView gets a larger height than the ContentView, the ContentView is 60pt in height(which is the same as setting in UITableViewDelegate), but the CustomView inside contentView is 76pt in height.

By Xcode6's view debugging, I see some strange constraints on my custom view, they are:

self.height = superview.height + 16

self.midY = superview.midY + 8

Where are these constraints come from and how to modify them? I've never set anything with a value 8 or 16.