In any organization in which rank may be attained it is inevitable that upon attaining such rank, the individual will begin to assume the rules are for everyone else but not for him. This was, in my 30 years of experience, just as true in the WT organization as it was anyplace else.
Therefore I'd expect, as the anecdotal evidence unfolds, to learn that GB members have many "legitimate" ways to avoid field service. They can merely cite the other important demands on their time.
This was certainly true of elders in the congregations I was in. Very few elders were really "active" in the field ministry. They were very good at creating the appearance of being active, but how often did "pressing congregational business" take them away from field service?
Rank has its privileges. Nowhere is this more true than among JWs.