The Terrace Cafe and GDR are sized with the assumption that many more folks will be dining in specialties than is normal on other cruise lines. When that doesn't happen (most frequently on an embarkation day) the crew will actively seek specialty volunteers to balance the load.
On the other hand, a 7-day cruise on a 4-specialty ship means they've guaranteed pax more days in specialties (4) than in TC/GDR (3), so there are few unclaimed specialty slots to hand out, even after they've made extra slots by starting service at 6pm instead of 6:30pm.
On a sold-out 7-day cruise on the 2-specialty Regatta, we had a choice of restaurants when snagging an extra specialty visit on Day #1, and no chance, no how, of anything after that.