For 11 of our 13 cruises we have used the lot and a long time ago they had just a lot. The parking structure didn’t even exist yet, if I am recalling correctly. Since the surface lot could accomadate the classic ships there is probably ample parking with the garage. We too have noticed that at times it is easier than others. So it just may be that you were on a cruise with more travelers that had to fly in.
The other factor is there is more than enough parking. I’ll guess that the ratio of people parking at the
DCL port itself may be lower than one might expect. If a Dream class ship holds a maximum 3500 guests it probably works out to be fewer cars. Let’s say DCL averages 3 guest per car. (Likely it is more since there are a number of family and larger groups that might have more people per vehicle, so my estimate is probably on the low side.) That means would they need to park 1167 cars. If everyone brings their own car! So at peak out of state travel seasons the number people flying and taking other transportation is probably greater than 50%. At 50% that leaves only 584 cars to park.
The premium lot has 113 or parking spots and the standard parking garage has at least 1000. Based on the number above, the parking would only be half full if everyone parked at the terminal. That number gets further reduced since there are cheaper off site parking areas with shuttles and hotels that offer stay and park options.