I can't see how this could have happened

If the story is true, metal plate in head is the only logical thing.

Yeah, I'm there with you. I could possibly see this happening for the mouse-gun calibers that would probably be carried by these punks: .22, .25, .32, or .380 could possibly ricochet off the skull and find soft tissue close by.

If it really did happen as they say, it was a freak thing.

"But then the round struck the man’s face, ricocheted and struck Chapman, killing him on the spot."

The story doesn't say what it ricocheted off of. Could be a glancing blow to the face, hit the wall behind the victim, ricocheted off that, and killed the criminal. Still a freak thing.

.22, and on the ricochet hit a really soft target like into the neck and a major artery?

I'm still betting on a bounce off of a hard object instead of the intended victim's skull? That makes sense for a 38 or better - glancing blow, hits concrete, comes back?

It is possible for bone to be extremely hard and dense.

A prize fighters bones of the face are hardened due to repeated minute fractures that later knit together and form a more solid surface. There are also some disorders that can cause bone to be nearly as dense and hard as steel. This coupled with angle and placement could have cause a ricochet of a small caliber round. If that round were to strike soft tissue it could cause death, the eye or neck would be two very vulnerable area's but neither would likely cause instant death and the chances of this happening is extremely remote.

http://www.sfgate.com/default/article/SFPD-Teen-was-shot-dead-by-fellow-robber-5097201.php

From the linked story:

"The incident happened Dec. 14 near the Alemany public housing complex in Bernal Heights. Police said a group of suspects accosted a man at about 11 p.m., and that as they dug through the victim's possessions and took his phone, one of them pulled out a handgun.

The victim did not resist and complied with all demands, police said, but the gunman still opened fire. The bullet, though, glanced off the victim's face and struck Chatman, who was pronounced dead at the scene, officials said."

Must be low angle of incidence, probably unusually hard skull. As Chris quoted, it was a glancing shot. Maybe bullet type was more prone to ricochet: I'm thinking rounded FMJ, not hollow point or SWC.

but I like the comment what goes around comes around and yes John Lennon was very appropriate :)

They don't say where Chatman was standing. Perhaps he was pretty much in line with the intended victim? It was a group of suspects after all.

I'm with KCSteve here: I'm thinking that the person who fired was roughly in a line with the victim and Chatman, as criminals really aren't well known for their fire discipline. Perhaps it was a glancing blow to the victims face, and Chatman happened to be in the line of fire.

See: Circular Firing Squad.

I've had several clients bounce rounds off skulls. Last was a marijuana deal, I frame Smith .32. One of the six rounds into the skull, muzzle contact (first one), bounced right off.

I had an uncle with acromegaly, his head and hands were affected and the bones in his skull had thickened. I suppose something like that could deflect a larger bullet, but like others have said, normally it would have to be something small, like a .22.