Just a more efficent OS which needs less resources. They did the same with the PS3.
http://www.engadget.com/2010/02/23/70mb-of-additional-ram-unlocked-for-ps3-developers/

It's fun to speculate, could be many things.

OS might be more efficient now, or maybe the extra cpu was held back in case tv, dvr, snap, internet browser etc, became very important. Same with all the memory resevered for the OS. Since the strongest selling point is games, the reserved resources can now be made available.

The less the OS does in the background the better anyway. With only 1 slow hdd to share, anything the OS does has an impact on the game you're running.

Back to the carpool lanes, 2 lanes were available when the highway opened. Not enough people were interested in car pooling, thus the extra lane was better used for the general public. Same with XBox One where reservations for Kinect proved to be pointless.

It would be interesting to know what kind of plans they might have had. For what the OS does now you don't need 2 cores. Doesn't the ps4 have an extra low power chip already for background downloads? http://vr-zone.com/articles/second-ps4-processor-extra-gddr3-ram-revealed/64081.html So what does the OS do while a game is running.

I thought the secondary chip to do background tasks while in standby and for "Share" features was announced in the Feb '13 PS4 reveal. Not 100% on that.

But yes you are right, could be a number of reasons. Optimising the OS could be one and they've done it in the past so it wouldn't be a surprise. The OS was probably given a wide birth to make sure the system to run smoothly when extra features were added in the future. Now they have a good idea of what they need they can start to chop down the resources.

The OS will handle stuff in the background while playing games like apps, messaging, chat, trophies. suspended apps etc and the UI has to pop up within an instant when called upon. My PS3 UI performance took a hit when they allowed more resources to developers so hopefully they don't try and be to tight with the PS4 OS resources.

OS features that should be segregated:

Trophies
Party Chat/ Messaging
PS Music/ Media Player Music
Notifications
Downloading
Navigating around the OS (including PS store)
Screen shots
Game play sharing/recording
Streaming to a remote play device
Internet Connection Maintanence
Audio streaming (to the DS4/ Gold headset)
etc

The gap was reduced, and this update will widen it again. 

None. The 2 cores was for futureproofing, Sony went over zealous with memory/processing to prevent another PS3 ui debacle. Insiders only claimed it was ever using one core, I'm suprised it took this long honestly. I figured they might be saving it for VR, but it looks like that's not the case.

If they use resources dynamically they won't need to intentionally allocate resources, but would be more difficult to code I guess.

So UI/OS would use that one core instead of two cores now, or that extra core was just for headroom if any games or apps running. Big deal in deed.

The later.

In all honestly it was a small boost to the Xbox One, and it will be a small boost to the PS4. It's nice to have, but it's only going to be a minor improvement just like the Xbox One.

Release the samba!