The last couple of CVNs of the NIMITZ class from CVN-76 onwards have had only three 'cross deck pedants' (yes I know but I make a yolk yoyce)
There are official explanations out there that I could find but IIRC some advantages are: less maintenance, less expense replacing on three wires and fewer personnel required. There are diagrams that may mean something with an explanation but that will take time.... Also these are AAG Advanced Arresting Gear which is being used here for first time. Does that help?
"The system provides significant benefits over current recovery systems; Operational capability to recover projected air wing, with renewed service life margins; Full compatibility with CVN 68-class and CVN 21-class carriers; Higher availability; Self-diagnosis and maintenance alerts; Reduced manning and Total Ownership Cost (TOC)"
http://atg.ga.com/EM/defense/aag/index.php
In the over deck view above one may see that the third wire has two sheaves (one vacant) in close proximity. In an emergency the barricade would be rigged on the vacant sheave &
"...The AAG system consists of four units, where a unit is defined as a single recovery wire and associated equipment. It is envisioned that the AAG deck configuration will utilize a “3 + 1” recovery wire configuration, where a maximum of three recovery wires are rigged on three of the units at any given time. The remaining unit may be utilized as a spare, enabling a recovery wire to be rigged in the event one of the other units becomes unavailable...."
http://www.globalsecurity.org/milita...age-i_2002.pdf
'35_aoa'[USN Super Hornet Pilot]: “FWIW,
CVN 76/77/78 3 wire boat hook to ramp clearance is (IIRC) more like 10 feet. It is different enough that our brand new CAG paddles on cruise (who were both 4 wire boat guys) spent some time learning the "new" sight picture. For a month or two, they were calling low all the way for what were on-on passes. Then the airwing paddles collectively got into the debate of whether or not an "OK 1 wire" was a thing. Technically speaking, it is, based on the reduced hook to ramp clearance, but it took several months to convince CAG paddles of this. Old habits and sight pictures die hard I suppose.”
http://www.f-16.net/forum/viewtopic....t=hook#p353647
OK two wire! Ronald Reagan (CVN 76) ramps up new technology. 01 Jul 2002 Dan Ball
“In 1995 Newport News Shipbuilding engineers began designing the ninth Nimitz-class nuclear-powered aircraft carrier, Ronald Reagan (CVN 76)....
...There are several changes on the flight deck of Ronald Reagan. A new design layout extending the port side angle of the landing area has moved the foul line clear of jet blast deflector two. The carrier can simultaneously launch an aircraft from catapult two and trap on the landing angle. Another visible change is a three-wire arresting gear design instead of the traditional four-wire system. The number two wire, located in the same spot as number three on other carriers, will be the "hit
[arrest target] wire."
The new system uses polycore cables designed to withstand more traps than steel cables and extra-large pulleys to reduce maintenance and man-hours, and provides the capability to land potentially larger and heavier aircraft.* The former setup of four arresting gear engines and one barricade engine is now four arresting gear engines [1,2,3 & 3A] with two of them interchangeable as barricade engines.
The removal of one engine greatly frees up the space to flight line maintenance crews...."
http://www.thefreelibrary.com/OK+two...gy.-a090332253