The first pass, reading the whole disk, takes a long time (often hours for HDDs), so you'll have to deal with a lot of modifications.

As long as the system was only under normal near-idle load, there will be some modifications, but not too many, so you'll be able to catch up much more quickly, leaving the window for new modifications even smaller.

Of course, if the disk is continuously receiving a high write volume, the race might never end. But if it's idle long enough for you to do a sync and a final collection pass before the next write, you'll have a full image.