The polarity of the contacts when wiring an EMR
When a relay is operated under DC conditions, there is material transfer between the contacts when they close and when they open. Without the presence of an arc in the contacts, the material transfer is always from the anode to the cathode. This can form uneven surfaces (asperity) on the contacts that resemble spikes and craters when examined under magnification. But this is normal for any relay. This is sometimes called "metal bridge transfer".

When arcing occurs on the contacts, the material transfer is in the opposite direction - from the cathode to the anode. And most relay applications involve some degree of arcing. This can result in more material being transferred than under non-arcing conditions.

Since the ability of a relay to extinguish an arc is based on the distance between the contact points for a given DC load, the faster that this distance is traveled by the moveable contacts, the sooner the arc is extinguished. And the acceleration of the armature is dependent upon the mass of the armature. It is therefore a reasonable conclusion that the recommendation ensures the net material transfer results in the stationary contacts gaining material, thereby reducing the mass of the armature contacts in order to achieve an increasingly faster contact switching time.

In any case, notice that the app note is a technique to obtain a longer than specified life from the relay. It is always nice to see such insightful tidbits.