Here is a stab at explaining the "average out" part of the question, hopefully tying together some of the points brought up in the other answers and comments.
Satellites that are not undergoing control of their orbital inclination will gradually adopt both an inclination that increases up to ~15 degrees (and after a long time, back down to 0 deg) and also a progressive change of Right Ascension of the Ascending Node (RAAN). This is driven by a combination of lunar and solar perturbations.
It doesn't matter whether they are at geosynchronous radius or in the normal graveyard, they are all affected. Operational satellites have to expend a lot of propellant to remain geostationary. As an aside, whilst re-orbiting (graveyarding) practices have improved over the years there are hundreds of abandoned satellites in the operational zone.
It is easier to visualise the combined behaviour of inclination and RAAN by imagining that the satellite's orbit is a spinning top with an axis at the middle which we'll call the orbit pole. In a controlled geostationary orbit the orbit pole is closely aligned with the Earth's spin axis. In a drifting orbit this pole traces out a circle whose own centre is ~7.5 degrees from the polar axis over a period of ~56 years. The inclination and RAAN can be thought of as the magnitude and direction of the orbit pole (OK, but for an inconvenient 90 degrees which I will gloss over, it's just a metaphor). Thus the maximum change in inclination is from 0 to about 15 degrees.
The key point of the question was, as I understand it, "why are all these satellites 'in phase'"? It is because they all adopt the same spinning top motion, i.e. all the orbit poles are in the process of tracing out the same big circle offset from the Earth's pole in the same direction in inertial space.
This means that at any instant all those satellites at similar longitudes will be at the same point in their approx daily cycle of latitude, north or south of the equator.
For all those that, at some time of day, are at their maximum latitude:
- 90 degrees of longitude around the equator there are others at 0 degrees latitude
- and another 90 degrees further away there are more that are at there greatest negative latitude
The idea of it being related to the Atlantic and Pacific however is just a co-incidence of the snapshot. The orbit pole motion is relative to inertial space under which the Earth turns regardless.