I've had that experience too, but it's become increasingly clear to me over time that the causation flows the other direction. The sprints that feel more productive are the ones that happened to have a lot of break-down-able tasks; the sprints that feel less productive are the ones where you have an important but hard problem with no clear solutions or break points. That all works out fine for teams that have to do a lot of greenfield development, sure, but does it reflect anything other than simple ease of planning?